Ok, history lesson time.
GWR (previously FGW and FGWL) don't contract Heathrow Express to run the Connect service. Heathrow Express want to run the service, but don't have the necessary track access or station safety case. Way back in the mists of time HEX approached FGWL, stating their intentions and, knowing FGWL has 2 unused relief line paths in their track access agreement and the relevant safety case to call at intermediate stations, proposed they jointly operate the service. Heathrow Express own the trains, employ the drivers. FGWL (now GWR) do the paperwork.
They're not contracting HEX to do anything - as I said before, the 360s are HEX's trains, if they want to run them on the Express that is entirely up to them. GWR have no obligation to cover HEX's missing services, and can't beyond Airport Jn anyway. However they've elected to attempt to run an hourly 5 car shuttle to Hayes in the Connect path despite stock shortages and an ongoing driver dispute, to try and alleviate the overcrowding HEX's decision has caused