What terminal are you going from?. They're only at T4 I think.
Here's some info on them:-
Location: London Heathrow Airport
Position: Terminal 4 next to Gate 15/16
On the Door price:
(Subject to space being available)
Adults (16 & over) £20
Children (2-15yrs) £10 - Children under 16 must be accompanied by an adult
Children under 2yrs - Free entry
Opening Times: 05.30 - 22.00 hours daily
You would need to book it in advance.
www.holideck.co.uk is the website.