It's not guaranteed for any specific dates but definitely worth a look.
Useful bargain rates, discounts etc for the Sofitel at LHR T5
The Premier Inn T5 is a great little hotel and cheap for an overnight stay and it's very easy to get back to T5.
The 423 bus, which is free, stops outside the lower area where arrivals is and it's 2 stops with the actual bus stops either way right outside the hotel and is very simple to get back and forward from.