Apologies if this sounds a bit negative.
Most people aren't going to travel more than a 15-20 mile radius on a regular basis, due to fuel, race fees and just not been able to get there in a reasonable time.
With the outdoor season, racers are concentrating on these meeting.
Friday nights is when other clubs race, so changing to a different night may help, but then this could have a worse effect as people have work or school the following day.
It would of been nice to see more one off open meetings over the Summer, the revenue off this would of helped cover rents. Sadly though every weekend is tied up with Tiverton and Worm, having there own race series on a fortnightly basis. Shame you couldn't of taken advantage of the two recent Bank Holidays.
Open all day on certain Saturdays for open practice may work, but I can't see numbers been that high with outdoor racing on a Sunday and it would probably only have an ill effect on Friday nights.
To be honest I don't think you can rely on the regular racer, due to many of the reasons above.
A local model shop where people can chat is always a good thing, you get people through the door, the interest starts to develop and some start racing.
The only way you are going to make the club grow is to enlist locally through schools etc. New blood will always support a local club because that's what they want to do, is race every week. And if they start racing, there friends start racing.
Hopefully you can pick some positives out of my negatives