Toulouse to Barcelona?

It can be done by train, maybe a six or seven hour trip. And, I suspect there will be a line change along the way. Somebody will come along in a minute and tell you exactly how to do it.

There are two options available....both early-morning departures.
The first leaves Toulouse at 617 am and arrives in Narbonne at 805, then leaves Narbonne at 820 and arrives in Barcelona at 1148 am.
The second leaves Toulouse at 654 am, arrives in Narbonne at 807 am then picks up the same train to Barcelona, arriving at 1148 am.
The regular second-class fare is about 66 Euro per person, and you won't get discounts on the first journey. But the second one uses the TGV on the first leg to Narbonne, so you may be able to find tickets on your travel day at abut 59 Euro.
You can check schedules and purchase tickets up to 90 days out at tgv-europe.com. Just keep telling the website you're from "Great Britain." If you don't get the "print at home" option, you should have the option of picking up your tickets at the station, with the credit card used to purchase them.

I've tried to find out if there is bus service - but I've been unable to find a user-friendly site in English.

With the change in trains (about 15 minutes in Narbonne) is that fairly do-able? The Eurolines site lists the price over 100 euros per person - am I not looking at the correct link?

Sure, 15 minutes to change trains is a lot of time....unless you've got several heavy suitcases to carry. Just be ready with a strategic grab of your luggage as the train pulls in.
According to the Eurolines Spain website, you can get from Toulouse to Barcelona for 35 Euro o/w pp on an exploratory date of Feburary 10th. One bus leave at 915 am and arrives at 330pm...the other travels overnight, leaving at 1130 pm and arriving at 545 am. If you can follow the Spanish, you SHOULD be able to print out your tickets at home. The French site is down for maintenance until at least 9 pm ET.

As well as the train routes quoted via Narbonne there is also the more direct but slower mountain rail route via Latour de Carol. This does not show up on most rail sites because the section from Latour to Barcelona is regarded by Spanish railways as part of the Barcelona suburban network despite the distance (this also means that tickets can't be booked in advance, but don't worry about the train being full). On a weekday typical journey times are

Toulouse depart 0850
Latour de Carol arrive 1152
Latour de Carol depart 1315
Barcelona Placa de Catalunya (at the top of Las Ramblas) arrive 1631

You have to change at Latour de Carol as the French and Spanish lines have different gauges. It's a longish journey but the trains are comfortable and the scenery through the Pyrenees is incredible (it includes the highest point and steepest gradient on French railways).

I posted a similar question a couple of months ago concerning rail travel from Perpignan to Girona, which is on the same line as Barcelona. I got a great reply from Tim in Minot N. Dakota, who refered me to a german rail website.
I'm going to look for it now, but I only have a few minutes (cos I'm going out), so if I don't get back to you with more information, maybe have a search.

Found it, it's on page 5 on this thread, titled 'train from Girona Spain to Perpignan France' (may be the other way round).

Tim (who, IMHO, is a genius) refered me to www.bahn.de

hope that helps!

I took this route last September and took about 7 hrs. The schedule that Philip mentioned was what I took. From La Tour de Carol to Barcelona was about 3 1/2 hrs and cost me only 8.95 euros! You buy the ticket from the conductor in the train. The best bargain I had on my trip. It's a regional train and I took the stop at Placa de Catalunya where my hotel was. It will go on to the Barcelona Sants station.