The last Heathrow Express leaves Paddington at 2330. I think you're going to be pushing it to get from Euston in time. Only a short delay with traffic or the tube could easily make you miss the train. Instead I'd simply catch the tube from Euston directly to Heathrow. There's a variety of options, but probably the best way to travel is to catch the Victoria line south to Green Park, change for the westbound Piccadilly line - direct to all terminals.
According to
www.thetube.com - this journey should take around 70 mins and will cost you £3.70.
Richard
[This message has been edited by rharrisuk (edited 06-25-2003).]