Paul
Most track days have instructors available and costs vary. Tuition is normally taken in 15-minute stints - any longer and you'll overstress your car and your brain!
One way to get the best out of it is to set a target - such as minimising understeer - and focussing on ways to deal with it. That way you learn technique and get faster at the same time
I find one period of instruction per track day to be ideal, but find I do have to concentrate exceptionally hard to put into practice what I've learnt. It's very easy to get into bad habits on a circuit!
If you're serious I'd say do a day with one of the better track day organisers, like
Motorsport Events, and book some tuition. They also do Academy days which are focussed on training
Don't let the mods to the car, the size of the engine or anything else deter you. Most track day regulars are only too aware that they're likely to be passed very quickly by a car they previously dismissed as being "slow", simply because it's being driven by an expert
Nick Froome
www.bolide.co.uk