I have sought some advice from a solicitor friend, and the letter has been sent, after some modifications he made to it.
On a related matter, here is a post I received from the Consumer Action group that relates to the sticky topic - crock of ****. You may find it interesting.
-----------I think that this is going to be a hard case for you to mount, not impossible, but hard. The tests you need to meet under the SOGA can generally be limited by the price paid and the expected quality of goods of that type; by that I mean that the standard you might expect of a new Landy is not that you will expect of a ten year old model. In many respects, you almost need to expect some problems. Sometimes what is said in an advert is a condition or term of the contract, sometimes it is a 'mere puff' and there is no right of redress attached. The trader might simply argue that the vehicle was in 'excellent condition' for its age and type, not as might be compared to a model with half the mileage and only 3 years old.
Even if you win, the having it repaired might go out of the window as it is disproportionate to the purchase price. I think that the argument might fly, as there is a 25% difference, but it is something to take into consideration.
I also think that the argument should be that it is not of satisfactory quality because it is not free from defects or that it is reasonably durable, not that it is not fit for the purpose. It is, it just needs repair. If you'd bought it to seat 10 people, then it would not be fit for the purpose but as it is, with repair, it is.
--------------