I have a cox's in the back garden, £3.99 from Aldi....unfortunately I lost the label, and it's very much like Elstar (another I bought at the same time, 3 yrs ago).
If I have this right, it's not difficult to grow, but the Elstar has been more productive (or it could be the other way around... ;D). I also have a Granny Smiths (again, bought and planted at the same time - would actually be spring 2006)
I would say, don't be put off.