This is a bit late (to say the least) but if anyone is still wondering about offroad tyres for the Beinn 20 then they might want to look at Schwalbe Mad Mike 20×1.75. My LBS got them in when the ETA for Conti Explorers kept being put back, £11 each.
I had bought Schwalbe SV6 inner tubes (intended for up to 1.5″ as he runs Marathons for the road) although SV7 are the size Schwalbe specify for 20×1.5-2.0″.
SV=presta, I don’t think the schraeder tubes (AV)will fit through the valve hole in the rim.
For the really keen Schwalbe also do a Marathon Winter in 20×1.75″ for £40 each.
BTW the Beinn’s 20″ = ETRTO 406, whereas some 20″ tyres are 451.