I think it would be better to keep the track day at Llandow, lower cost is always going to be important as very high numbers can never be guaranteed. Being in Leeds I'm about the furthest away and the distance really isn't a complete obstacle, its about a 4.5 hour drive.

Some of the guys up North could get organised and hire a van / share the driving.

The campsite next door is very handy. Also, it may be possible to block book a local hotel in Llantwit Major or similar. The place I stayed at was £50 per night, and I was one of only 3 guests! If we said we'd fill the hotel for the night before or after, I reckon you could ask for a serious discount, maybe £30 per night, which is hardly more than the campsite and there's cold beer on tap!

And to get numbers up and costs down, I personally wouldn't be against 400s being on track

And there are so many other track days all over the country on such a regular basis, including SATT at Cadwell, there is no reason why other 'regional' gatherings couldn't be arranged as well.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30