Thanks for the offer SixSpeed, but I’m about 400miles north 
Have ordered the stainless steel one from MX5 Parts, getting the crush washer, nuts and bolts, and hangers from MX5 City as they were considerably cheaper than MX5 Parts for those parts but a wee bit dearer for the backbox.
I’ve tried the WD40 Penatrting fluid - no joy - so I’m about halfway through sawing off one bolt, looks like there’s a flange gasket, so If I can get that moving it should (hopefully) make getting the other one off - though think that’ll need cut off as well, to be honest. Hoping for a dry Saturday - not so sure about the Sunday - want to watch the footie