Thanks for the reply Matt (seems you're lucky to get one on here),
It turns out I was given the wrong filter (T12203 in GSF cat). Although it was the one listed in the catalogue for my year of reg., mine is one of the later 205 1.4's and takes the same filter as the 106 Inj (T12205). They probably changed it when the 106 came out to streamline supplies.
It's an oval filter, open at both ends. The usual filter (the one I was given) is sealed at the base and comes with a black plastic casing stuck to the top.
As all the pipes were undone, I didn't want to put it all back to get the right filter from GSF, so I walked round to my local accessory shop to get it.
He looked up my make and model in his catalogue and brought out exactly the same wrong filter, so it's obviously a common mistake.
Eventually got the right one, but it was £5.50 compared to only £4.17 at GSF (£8 in Halfords).
I'll probably be asking a few more questions over the next few months, so keep looking in.