So, tackled this job yesterday, so quick update: Went with existing plug washer, seemed in reasonable nick. Though totally forgot to measure it whilst it was off, Doh!
Old coolant that came out was a dirty brown sludge, really rank. I presume this isn't normal or particularly good... Ran the two part Rad Flush stuff through, along with numerous flushes through with tap water, eventually came through clear, but a lot of bits and crap came out along the way.
Just used deionised water in end, weathers been too dry to get anything from dehumidifier. Mixed it with some Halfords OAT concentrate, went for 50/50 mix. Didn't quite fit in 1.95Ltr as book said, was up to upper mark after about 1.6-7ltrs odd. Did the usual tricks to free any bubbles, squeezing pipes, shaking bikes, blipping throttle etc. Will just keep an eye on rad filler and see if it needs topping up in next day or two after I've been out a bit.
With such manky coolant coming out, if there anything else I should watch out for?