I checked the internet for pictures of a plaice and what Keith pulled out was definitely the same as the pics i found.

Not being funny but if you're basing identification on internet photo's I'd definitely say it was a flounder with orange spots (given the river is full of them at the moment), I'm not ashamed to admit from a photo I'd be hard pushed to tell the difference between a spawning (bright orange spots) flounder and a plaice, I've chucked good plaice back thinking they were flounders because the spots weren't prominent enough to later discover the spots are only an indication not the definite.

As I say I'm not taking the pee or being awkward, telling the difference isn't that simple, you could put a plaice and a flounder in-front of a lot of anglers and they would take a wild guess and if one had more prominent spots than the other they'd point at the spotted fish being a plaice which isn't necessarily right.

Some pics of spotted flounders on another fishing site LINK

This is a toppa bit of info on the difference between flounders and plaice from same site, well worth a read! LINK