Surprisingly slow.

1) Bus 482 and 490 from T5 bus stop 8 on the Arrivals level - that's approximately half way along. It's free within the complex but you will see people using their Oysters for offsite destinations. 15 mins, service every 6 to 10 minutes.

2) Heathrow Express, but change at T1. It kind of connects, but it could easily take up to 30 mins to do this.

3) Hilton T4 bus if you are feeling sneaky. Bus stop 18 - furthest end from Domestic Arrivals (or underneath F check-in if that's how you know it). Also 15 mins but it doesn't run regularly.