DMS is probably your closest option. they race Thursday nights and there a few sunday events too (mainly regionals i think)
There's newbury too, dunno how far that would be for you though. im sure some others might know of more indoor in that area
Slightly further afield you've got HNMC up in Ware. its outdoors on tuesday nights in the summer and our winter is currently running 1 sunday per month
HTH!