Train from Euston to Watford Junction on Monday August 14th

The Engineering work is only affecting the London Overground Lines (the 'slow' or 'DC' lines) on what is a multi track railway. London North Western/West Midlands (and Avanti for those travelling further than Watford Junction) trains are able to run on the other parallel lines as normal, and are running a full normal service.
You wouldn't have been using London Overground anyway as they stop at "every lamp post" and take an eternity.
Yes just turn up at Euston and buy your tickets. Other than convenience, there is no cost benefit from buying tickets in advance on that route.
People use the Overground for short journeys, not throughout journeys. Those short distance journeys are being provided by replacement bus services, so I wouldn't expect the LNWR trains to be significantly busier than normal.

Thanks!! Appreciate the prompt response!

Cool, I'll be going to the HP studio tour the same day!

You can travel to Watford Junction with contactless or oyster card. If you will be travelling in London, part of the journey might count towards the weekly cap.

If you have a railcard, the off real discounted fare may be lower than fare charged on oyster or Contactless

When you arrive at Euston you will see a big board listing all the train departures. At the top of the board, you will see the ultimate destination of each train, and then there’s a list underneath of all the places it will be calling at - that it, the stops en route.

You are not looking for one with the ultimate destination of Watford Junction - although it sounds like they’re not running anyway.

You need a train where Watford Junction is the first, second or third of the listed stops. From memory, the ultimate destination might be Tring, Birmingham, Northampton or Milton Keynes - although not all trains to these destinations will stop at Watford Junction. However, if they stop there, it would normally be one of the first three stations listed on the “calling at…” section.

This pic might help. You’ll notice the Northampton trains here list Watford Junction. The ones with the ultimate destination of WJ are the Overground ones and even if they’re running, avoid them as they take so long: https://flic.kr/p/86rTBG

NB that photo was taken in the middle of the night. You’ll see far more trains in the daytime with a greater variety of ultimate destinations.

Golden Girl's picture is out of date. Those central screens above the platform entrances are now switched off. There are now banks of LCD screens in the concourse, with lots of colours- like the ones at Victoria Train Station.
They now have a lot more information on them. Some people complain of information overload. If in doubt ask one of the very visible members of staff.
On each of those banks there is one screen which specifically gives the next departure for Watford Junction. There were teething issues with them showing an Overground train rather than a long distance train, but that should not be an issue if the Overground is not running, and hopefully the glitch has been overcome.

This web page gives a general overview of what the new screens look like-https://www.networkrailmediacentre.co.uk/news/high-definition-departure-boards-for-euston-station-passengers

Above each platform entrance there is a repeat screen for that specific platform, which also gives a countdown to when the platform gates close (in seconds)- generally two minutes before departure.

Crikey - sorry for being out of date! I used to use Euston at least weekly but moved away from the Midlands two years ago so they obviously changed things.

Thanks for the update and ignore my post.

There is an update to this, having been to Euston today.

The old screens are now being dismantled so physically do not now exist.

There is also a set of the new screens on the piazza outside the station. On each of the three sets of new screens the next Watford Junction has settled down to being the next Overground train.
However there is also an alphabetical list of stations served with next train for each one. On that list the next Watford Junction is the fast train.
So tonight at 1650 the next Overground was at 1657, and the next fast at 1704.
Then you look for a screen showing a 1704 train. That lists all stops. So the 1704 was to Tring, first call Watford Junction.
For avanti long distance trains it will also show you how full each car is.

I am surprised that the Tring trains are first stop Watford Junction. When I worked that line as recently as 5 years ago, it was always Harrow & Wealdstone, Bushey, Watford Junction, Kings Langley, Apsley, Hemel Hempstead, Berkhamsted, Tring. Probably because of Peak Time service.

Back in the stone age it also stopped at Queens Park but that was nearly 10 years ago...

In the peak all Tring services from 1605 to 1834 are first call Watford Junction.

In the off peak they are now (as opposed to years ago) Harrow and Wealdstone, Watford Junction, Hemel Hempstead, Berkhamsted and Tring.

Tring services only stop at the smaller stations very late at night, at times when no visitor will be travelling.

This is the timetable for the local stations services out as far as Cheddington- https://www.londonnorthwesternrailway.co.uk/media/2621/download?inline

but this excludes the longer distance LNWR services to points further north, which are on this timetable- https://www.londonnorthwesternrailway.co.uk/media/2620/download?inline

Together they show all fast and semi fast trains to Watford Junction from Euston, which shows the very high frequency.

In particular the LNWR semi fast trains to Crewe no longer call at Watford Junction.