sorry if i am being a pain but can you explain what the advantages crop format give on ef-s that are not available on ef. buying a 50d i have jumped in at the deep end

but i am enjoying learning and appreciate all the replies and joining in a great forum, by the way don't think i will upgrade in the near future, bri
No worries Bri.
Lens designers have to compromise. The common choices include focal length range, low maximum f/number and format coverage. In addition to optical performance, build quality, size and weight, cost etc.
Crop format is much smaller than full frame, about 2.5x smaller, and released from the constraint of having to cover the whole of a bigger sensor, you can do more with the other things like focal length range and low f/number.
A good case in point is the EF 17-40mm f/4 L, compared to the EF-S 17-55mm f/2.8 IS. While the EF-S lens is not so solidly built and even costs a little more, you get substantially more range, a full stop lower f/number and IS thrown in.
The lower f/number is a big upside. If you want f/2.8 in an EF lens, you have to go for the 16-35mm f/2.8 L which not only has even less focal length rnage, but costs twice as much.
Looked at another way, compare the EF-S 55-250mm f/4-5.6 IS on a crop format camera, with the 100-400mm f/4.5-5.6 L IS lens on full frame. In terms of effective focal length range and aperture, they are almost identical but one costs £200 and fits in your pocket, while the other costs over £1k and is a substantial lump. Okay, not a direct comparison in lots of other ways, but I'm sure you get the point.
And at the extreme wide angle end, it is just not possible (or remotely affordable) to produce a full frame EF lens with a focal length range like the EF-S 10-22mm.
The benefits of the smaller format quickly run out over about 55-60mm, which is why Canon only makes EF lenses in longer focal lengths.