Those bolt will have been there a long time and they may have corroded in well, unfortunately the bumpers are aluminium and the rail the bolts slot into will not stand the treatment you're about to put on it...the best thing to do here is the grind the nuts off. It's a bit awkward but manageable. I had both bumpers sand blasted and powder coated they look brand new now.
Assuming the bolts that hold them in have been removed then try the following. Remove the rubber grommet and liberally apply easing oil. Follow the chassis member into the boot and drill a couple of small holes to allow you to apply more oil along the length of the bracket. Leave for a day or two while applying more oil. Try to turn the bracket with the help of a large adjustable to help break the hold - don't go mad or you can easily bend things. Using a slide hammer to pull the bracket out will allow a decent bit of force to be used without the risk of damaging the bodywork
Thank you everyone
Finally got them out yesterday. They had been squeezed in really tight but a bit of perseverance paid off.
Most of the engine components are off now and she's ready to come out for an overhaul. Luckily my brother is a trained BMW mech so he is going to take engine apart and refurbish. Again any advice here would be greatly appreciated.
Can't tell you all how much I'm enjoying this process so far.