So, a quick update. Thank you very much to everyone who replied but particularly to Chris and Daniel who have both been unbelievably helpful.
It looked as though I'd be the next owner of the MacMillan Ribtec and I was following its progress with interest. However, on Friday I was offered a searider 5.4 at a price that was impossible to refuse.
As such, despite my earlier concerns about seariders I am now the proud owner of one!