Talks on Wednesday 26 February will be in Room D103 at 25 Gordon Street (which also houses the Department of Mathematics and the Students' Union).
Talks on Thursday 27 February will be in Conference Suite 3, 188 Tottenham Court Road.
See the map.
You might want to consult the Tube map. If you plan to use the tube a lot (on this or subsequent trips), it is probably worth buying an Oyster card.
If you arrive at Heathrow, take the Piccadilly Line to Russell Square for the Imperial Hotel or change at King's Cross/St Pancras to the Circle, Hammersmith & City or Metropolitan Line for one stop to Euston Square for 25 Gordon Street (Wednesday's venue). Either journey from Heathrow takes about an hour. It's not worth taking the more expensive Heathrow Express as you will lose time (and possibly the will to live) trying to work out how to change at Paddington.