Disco 2 reduced MPG

I've got a 51 plate disco 2, I use it to tow a horse box and just as a general work horse. It used to do about 26 mpg which I was happy with. But now it's doing something like 17, I haven't changed the way I drive or what I use it for. It's just had an MOT and they've done 'all the checks' and can't find anything wrong.

Has anyone else had this problem? What's the best way to the MPG back up?

Thanks in advance