Lucinda,
When my flight was from heathrow earlier this year think best i could do was around the same rate as you have been quoted.
However when i looked into it further was able to get a hotel/park deal for £65 by taking the "undercover hotel" option through holiday extras. You dont know the hotel before you book but i knew it was oing to be a 4 star and Diended up in the Park Inn. Told others on the same holiday and they got the same deal.
Didn't include transfers but was on the free bus route so used that.
Have used this method many times (i know it seems silly getting hotel and parking chaper than parking only)
Much more relaxed staying night before.
Mick