Just back from first family 'car camping' trip. Have a load of good Alpkit stuff, but went for a low-end Coleman sleeping bag for about £26 from Amazon because:
– I am generally very warm anyway
– We know we're only likely to go in moderate or good weather
We also got a good Mountain Equipment bag for the missus to ensure she didn't get cold and that I could use for proper hike or bike camping (as the pack size in literally half or a third the size of the lower cost Coleman).
So whilst I massively support the Alpkit recommendations (their mats and headtorches are fantastic, amongst other things) I understand where you're coming from and can recommend the Coleman sub-£30 bag.
Hope that helps.