It's a complete pain in the a$$, all those plastic clips along its rim need to be popped through, getting the first one is the hardest but I used two flat head screwdrivers to prise it open while my mate pushed it out. Then you have to prevent it from popping back in while you get the second out, from then on with a bit of force you should be able to work your way around. What way will your new grill connect? I got a black honeycomb grill off Ebay and only once I had the old one off did I realise it didn't just pop back in where the other came out...it had to be attached with screws (not provided with the grill which had me searching all over my house!!).
Edit: I just read that you have removed screws already, happy days, mine must have been changed before I acquired it and the screws must've been just left out! £90 is pretty good btw :icon_thumright: