any recommendations for heathrow to gatwick car service?

Just Airports is one service often recommended; I have no personal experience with them. I see that they don't list the price for this particular trip on their website, so you'd have to e-mail them: http://justairports.com/contactus.php

Maybe of some help:
http://www.tripadvisor.co.uk/ShowTopic-g186338-i17-k4886933-POLL_Airport_Car_Service-London_England.html

That poll on TA could be helpful. Just be sure to take the results with a grain or two of salt as it is four years old. Sometimes changes can happen in such a long period of time...

FYI - don't be tempted to think that a car can necessarily do the 40 miles in 40 minutes at an average 60 mph. There's often a queue onto the M4 extension, and can be quite backed up on the M25 on and off all the way to the M3 or A3 and then again around Junction 9 for Leatherhead. I would always count on 90 minutes and if I got there early it would be a bonus....

good luck next week. How much time do you have?

"as it is four years old. Sometimes changes can happen in such a long period of time..."

It was started 4 years ago but the later pages are more up to date.

Thanks.

This is all helpful.
And I've got even more time to sort it out. East coast snow event caused canceled flights, two day delay in getting out.

All is good. Just glad for the chance to travel.