So, referring back to my OP I think I have found the answer to my question
Today I attempted to fit a Clement USH 700x32 tyre on a Stans Radler rim using OKO Off Road Sealant which is about the consistency of custard. Using about 50ml of the sealant the inflated tyre failed to seal effectively, losing small amounts of air around the entire circumference of the beads. Despite every attempt to distribute the sealant by spinning and flipping the wheel, the situation didn't improve. I then added about 50ml of water into the tyre, spun and flipped the wheel as before and re-inflated and bingo the tyre pressure held first time.
By way of comparison I had previously used Stans sealant to fit the other tyre on the front wheel. Using exactly the same procedure the tyre would lose pressure from 50psi before settling at around 20psi over a period of around 6 hours. It took several spin, flip and re-inflate cycles before the tyre finally sealed properly.
So, it's my conclusion that bike specific sealants are purposefully thin to seal the bead to the rim on fitting of the tyre, and I'm surmising that a thicker sealant that is able to cling to the inside of the tyre will be more effective against punctures. So hopefully, starting with a thick sealant and adding just enough water to achieve a rim seal will produce good results.