dodgy second hand car dealership legal advice required

Can anyone please help me with the following query?

I have recently been looking to buy a second hand car, and after
visiting SxxxxCxx (not sure if I can legally name the dealership) of
Liverpool, agreed to buy a Vauxhall Frontera.

After negotiating with the salesman for a cash reduction, we agreed a
deal and a written agreement was made between myself and SxxxxCxx. The
agreement was that certain work would be carried out on the car, to be
complete before the car was collected. The date of the agreement was
Monday 10th November 03, and the car was to be collected on Saturday
15th. I paid £100 cash deposit to secure the deal.

I arrived at the garage on Saturday to collect the car as arranged,
after driving the car to confirm that the outstanding work was
satisfactory, I agreed that the sale would go ahead. The outstanding
balance was £4000 which was paid via debit card. A receipt for the
balance was issued by SxxxxCxx.

The salesman I had been dealing with left to obtain the vehicle
documents, but returned to say that there had been a mistake in the
advertised price for the vehicle and that they could not now go ahead
with the deal. The price I would now have to pay to purchase the
vehicle had risen to £5100.

Not wanting to be left without either the money or the vehicle, I
agreed that the £4000 balance I had paid be returned onto my debit
card. The salesman said that a cheque for the £100 deposit would be
sent to my home address on Monday (meaning 17th). As of today the
cheque has not arrived.

What is my legal position regarding the deal? As I am in possession of
a signed agreement and a receipt for the purchase of that vehicle, can
I insist that they honour the deal? I now believe that they have
subsequently sold the vehicle.