Interesting reading people's responses!

I live in Hertfordshire, so if going to T5 for work I'll have a driver take me costing ~£140 return. Time taken totally depends on time of day and traffic density. [touch wood] I've never been late for a flight.

If going for personal reasons I'll typically use the valet parking located immediately outside T5 Club check-in. I've often found prices competitive with with the offsite park and bus services. Most recently I paid £110 for 14 days. However I believe you can get similar prices with a hotel stay included at a local(ish) hotel.