We were told there is a train to Derby from that train station. Where is Pancras station from Heathrow
Thank you
You can look up train times and buy tickets on the National Rail website: http://www.nationalrail.co.uk/
London St Pancras to Derby, there are two departures per hour (leaving at xx:26 and xx:58), the trip takes approx 1½ hours.
And the simplest way from Heathrow to St Pancras is, as Emma says, the Piccadilly line (tube). Takes ~1 hour. Heathrow Express is slightly quicker, but involves changing to the tube or a taxi.
In other words, St. Pancras is in downtown London, and you have to come in from the airport with your luggage. It sounds like you may be taking a suburban service, but even if it is a long-distance rail product, be sure to download the entire timetable so that you can adjust on the fly, in case your flight is late, or your immigration line is slow-moving. It looks like Manchester might be a closer and cheaper international airport, if you still have the option (and the routing from your home city) to consider.
St. Pancras is a massive, renovated station with plenty of places to eat and (pay to) use the toilet, while you wait for your train.
Derby is not a commuter destination, it is Intercity. The trains run at up to 125 mph, which is why it takes only 1½ hours from London.
Manchester airport is closer as the crow flies, and it has a station at the airport. It takes about 2 hours from Manchester airport station to Derby.
A even better option is Birmingham Airport, 1 hour to 1h15 from the airport station to Derby.
The local airport to Derby is East Midlands. This does not have a rail connection, but may be close enough that you could afford a taxi.
Do be aware that the Midland Mainline station is relegated right to the back of the historic train shed and takes a while to get there from the Underground.
Another option is that there are a few daily long-distance bus journeys direct from Heathrow to Derby - check on www.nationalexpress.com to see if one is close to your arrival time. If so, it'll be more convenient than going by train from central London and won't take much longer in total.