If you looking at the Notts/Derby/Leics area, there are a number of tracks you could contact - some permanent, others semi-permanent.
The track you are probably referring to was Ardent Raceway which has sadly closed.
In the same kind of area, you have Broxtowe (just off J26 of M1) which is a semi-permanent facility but run outdoors once a month during the winter (this weekend is their next meeting). Broxtowe also run indoors on a Saturday night, less than 5 mins from the Outdoor venue but I believe this is more of a flat track for on road. You also have A1 Raceway just outside Grantham which is a permanent facility but you need to be a member to practice there. You have West Bridgeford who run a club night indoors on a Friday night which might be of interest to you as well.
You've already mentioned RHR, which is a top quality facility. You can usually arrange for ad-hoc practice there via James Helliwell (James on here) for a small fee.
Coventry is also worth a look as they have a great Astro track which is Permanent and available for practice to their club members.
You also have Silverstone indoors who run regular club nights midweek; check their Forum section for details.
HTH
