I do quite a bit of e-bay selling, but am in the U. S., but I think (hope) a lot of this will apply to you.
First of all, check out the discussion boards. They can be found bu clicking on the community tab on the top of each page, then click on discussion boards. They have lots of information, and like MN, have many people with lots of experience to help and answer your questions. That is by far where I learned the most about selling. I can't recommend them enough. The UK ones seem a bit slower in the brief second I looked at them. If they don't help much, check out the US ones. E-bay also has tutorials to help new sellers.
If you want to find out what your jeans have been going for, click on advanced search, type in what kind of jeans you have, and then check the box for completed listings only. That will show you what they have been selling for.
Many full-time sellers will actually tell you to stay away from reduced fee listing days. Unless the item you are selling is rare or in very high demand, everyone and their dog sells on those days, and sellers find that their items often sell for much less, or not at all because there is a flooding of merchandise on those days, so listing on those days to save a few pennies (or pence in your case) can actually cost you more than you save.
I think what Skribble is telling you to do is when you are listing an item you can quote several different methods and corresponding rates to ship an item. The first one you enter is the one that will pop up when someone is searching for an item, and the other ones don't show up unless they click on your listing directly. So enter the lower priced, slower shipping option first to reflect the lower price and draw more people to look at your listing.
Hope this wasn't a waste of your time, and it helps a bit.