Thanks very much for your response. I managed to ring the CAA today and spoke to one of their doctors. He wanted me to fax to him my optometrist prescription which I did. My worst eye is -3.25 (my other one -1.75) and I know that for a class 1 medical up to -5 is ok, however, I wanted to clarify the ATC restrictions.
Although the doctor told me that -3.25 will be no problem, he did mention that at the moment the upper limit is -3, so theoretically I would be -0.25 over, which is not good (although as I said, he didn't think it was a problem and sometime soon the ATC standard will be the same as the Class 1, ie. -5) - may not be soon enough though!
I will try and check with NATS again tomorrow to see what they say as they handle the applications...