I got to the bottom of it - Its flaming Britpart. They actually weld the bolts in from new! Must be a some cunning plan to make sure you have to buy two sections of exhaust, at least, every time. With my resources at home I was unable to shift them, even after angle-grinding the heads off - no fun lying on your back in the yard... so ended up taking it to Lix, as I knew they had the parts if required... easy enough job to knock out the "studs" after a quick blast of "Gas Axe"....and then round two started. The existing exhaust was all over the shop, bodged brackets, brackets swapped about and in the wrong place, all sorts. Initially any which-way we tried fitting the new exhaust it fouled something. So - new intermediate pipe, different tail pipe, swap a couple of brackets around and fabricate a new midsection bracket.... all lines up, no fouls and relative quietness reigns. Hoorah !. Thanks to John at Lix for getting me out of a hole there.