Hi Lloyd, I've used a Meyer ebonite piece in the past and at the time it really suited what I was doing. They are relatively free-blowing (for me anyway) and not too expensive. At about the same price you could try an ebonite Selmer, which are a bit rounder and less edgy-sounding than a meyer, but again very easy to play on. I had an Otto Link ebonite piece some time ago, and they are nice too, although for some reason seem to be noticeably quieter.
I also had a couple of those Rico Royal graftonite mouthpieces, can't remember which they were (they come in A, B and C 1, 2 and 3 making a range of 9), but they were ok too - I got them for about 20 quid in Perfect Pitch, Chesham.
The Vandoren Java mpc's also come recommende by some as an inexpensive upgrade from a basic mouthpiece, although I haven't tried one on alto (I use a Jumbo Java on tenor, nicely powerful).
As well as your local shop, and also if a trip to Croydon is too far or too time-consuming, there is Myatt's in Hitchin or Michael White's just off the North Circular less than a mile south of the dreaded Hangar Lane Gyratory. Hope you find something you like.