Use M8 x 20mm stainless hex screws,
I was out in the shed tinkering with the bikes this weekend again and I took a further look at this, and with the benefit of my glasses on, I took a closer look at what was going on here, and I was previously trying to secure them with M6 bolts with penny washers, which just weren't up to the job. Goodness knows why I hadn't realised this previously. Tightening up a set of M8 bolts felt much more secure, and I suspect that's the job done, hopefully nae more slidey seat syndrome!
Thanks,
Mike
