This is the article I'd read with regard to having it repaired at a garage of my choice:

http://www.cardealermagazine.co.uk/publish/court-rejects-dealers-right-to-repair-fault-and-orders-him-to-pay-2750/52909

And on this site:

However the implication for the second hand dealer is that unless the dealer defines a "returns and repair" policy at time of sale in their Terms and Condtions then the consumer has no need to prove there is a fault with the vehicle following sale, nor indeed is their any requirement for the dealer to be offered the opportunity to repair any fault or refuse to pay if the customer takes the car to his own repairer, providing that the costs of the repair are deemed by the court to be reasonable.

Your first thing to do would be to approach the dealer in a nice way before shouting legislation at them. They might be more co-operative than you think.

Your problem is that it is a 100k sub £4k car with a clutch issue. The dealer could simply argue that 100k is acceptable mileage for the components concerned. Cars have to be "satisfacotry" but that takes into account age, mileage, price paid etc. so you won't get anywhere near as far on this car as you would a 10k 2 year old one you paid £12k for.

The idea that all faults are covered for 6 months on older, higher mileage used cars is pure fantasy.

You could, by all means, get it fixed and then sue the dealer but I don't think you'd be onto a winner and if you lose you could land yourself with a legal bill many times larger than the cost of the clutch (many dealers have insurance for expensive legal representation).

As ever, that is only an opnion, and if you wanted to take it further you'd need to get proper paid for legal advice.