not worth it - a fortune - will be around 75-100 pounds easy. There is an Airport express something like 15 pounds one way into Paddington in 15 min - easier, quicker and cheaper. Then you can either get the Tube or grab a cab from Paddington.
I lived in London and used a minicab service called Keens +44 208 8754920. It will cost you 50 pounds to get from T5 Heathrow to Green Park/Piccadilly.
That's great - thanks very much! I thought it would be around the 50 quid mark these days, but it's got to be done as I'll be carrying too much stuff to lump around on the Heathrow Express.