Just to report that I purchased and fitted a pair of presta Dr Sludge tubes, also whilst rummaging in the attic I came across another couple of sets of MTB tyres, namely Schwalbe Land Cruiser which I think came from an abandoned work bikeshed bike a few years ago, anyway they claim to have puncture protection but are pretty weighty. So tyres and tubes are quite a bit heavier than expected, initial inflation (using a track pump) and sorting out pressures is trickier because of the gloop affecting the valve operation. Once riding they definitely take more zap to get up to speed. At ride speed all is good & I am impressed how quiet these tyres are, yet they grip very well on tracks/dry'ish mud. Not any slower ride-wise as my av speed was higher than before with this combo
I think I may reduce tyre pressures a bit next time because despite all the above positives the ride was quite harsh and wooden feeling. I was running both tyres at 50psi so I reckon dropping to 40psi or so should do the trick, any advice on this appreciated! I do think this tube/tyre combination should be good for the canal towpaths which is where I want to explore this year.