Sorry Gouabafla, saw your message too late. I have been using NCP lately when on business. The parking area is adjacent to the Park Inn Hotel, at the A4 end of the Heathrow spur. If booked in advance, it is 12.99GBP per day, 20GBP for 2 days. Bit of a challenge using the access road at present, due to alterations at hotel! Possibly closer to terminals than Pink Elephant. Best for terminals 1 & 2, no pick-up from terminal 3 at present, does not serve T4.
