Right so I'll explain from the beginning. At the weekend I fitted my EGR delete pipe but to do so I took the brake fluid reservoir cap off. I also thought it would be easier to undo the bolt holding it to the engine bay so I could maybe move it to give me better access to the bolts of the EGR valve. The bolt I had to undo to move the reservoir was a pain to undo so I got it half way and thought fook it and started to tighten it back up. This was also a pain so I got it half way and thought I'll do that up later.
Pic of the reservoir I fiddled with:

Contined on and fitted the EGR delete and took the car for a spin to check all was ok. I came to my first junction touched the brakes and panicked. The pedal went solid before I stopped and I had to push it really hard to come to a halt. So this morning I popped over to the garage and they said the vacuum was fine and working but he noticed the screw I had forgot to screw back up.

He said that could be causing a leak in the brake system or it could be a problem with the servo. So I booked it in for 2moro so they could have a proper look at it.
On the way home I thought about what he said and thought I'd screw the nut back up and see if it would sort it. Didn't think something that simple could work but low and behold it bloody did! Done it back up nice and tight, took it for a spin and the brakes were perfect again!

So the moral of this story is always remember to tighten everything back up fully :embarassed: