Hi Graham,
I hope you'll enjoy it. Having taken part in many track days, and also organised a fair few in my time, I have the following advice for you:
Car care:
Your car sounds as prepared as it can be fore the trackday. As you already mentioned. Check oil, water and tyre pressures before you go out on track, and regularly throughout the day.
Don't fill your fuel tank up completely. 3/4 of a tank is fine. You can always refuel. If you do fill it to the brim, you can have overflow with the fuel being pushed out of the filler under heavy cornering.
When you are finishing a session on track, have a slow lap. Race cars don't just do it for the adulation of the crowds (althoug hthat is nice) you need to let the car cool off a bit before you stop it. As far as possible, don't use the brakes on the cooling down lap. Drive around slowly and use the gears to slow down as much as possible.
When you come into the paddock and stop, DO NOT apply the handbrake. The brakes will be hot, and if you put the handbrake on, they will not be able to cool down evenly, so you can warp your disks. Just leave it in 1st gear. It won't go anywhere.
Driving:
Take it at your own pace. Don't feel pressured into driving faster than you're happy with. There will be some very fast drivers there, and people with a lot of track driving experience. If you try to keep up with them, you're likely to crash.
There are no prizes for being the quickest car out there. The only prize you want to take home is your car in the same shape as you arrived in it in.
Start the day like you were just driving on the road. Then gradually build up speed throughout the day. If conditions change (like it starts to rain) back right off and start your learning process again. Lines often change when the circuit gets wet, and also a race circuit can get a lot more slippery in the wet than the road would.
Be courteous. There will be a briefing at the start of the day where the rules will be spelled out. In genreal, cars will be allowed to overtake on the left only (although it sometimes varies). If you see a car coming up behind you, let him pass. Remember that some cars don't have much power, so they can't just floor it on the straights to get past. They are quicker round the circuit (otherwise they wouldn't have caught you up), so indicate, pull over and ease off the gas a bit until they get past. You'll get a cheery wave as they pass you, and they'll think you're a great bloke. If you floor it and don't let them pass, you'll get a different sign and they'll think you're something that rhymes with banker [
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If there is a car in front of you who isn't getting out of your way and is holding you up, don't try to be a racing driver and outbrake him into a corner, or pass him on a corner. Just drop back, let him get ahead, and then push on again. When you come into the pits you can have a quiet word with the driver, or if you don't have any joy there, speak to the organiser and they will have a word.
If you can get access to the Motorsport News guide (used to be called the Aurosport guide, and a lot of people still refer to it as such) have a read through the section on Croft. It'll give you a few ideas about what to expect from the various corners.
Most important of all.
Enjoy yourself. Croft is a great circuit.
Cheers
James