I can check the thread on mine tomorrow night, but yours may be different, the liners are 4mm so push fit fittings would be ok, although the fittings do vary in quality, I've bought cheap ones off ebay in the past and they are almost impossible to remove the old nylon tube
forget about looking for the fittings and look at what you're trying to achieve, where there's a will, there's a way. these are the easy options
Plan A
you might be able to get one from Clarke but their postage charge is £6.75 + vat.
Plan B
you can get cut the end off your liner and use some 6mm bore tube over it,and hold it in place with cable ties (but not glue as it doesn't need to be gas tight) like I did for my gas line and use that to join the two tubes (can use airline tubing from any pet shop)
Plan C
cut the end off your liner and get a 4mm push fit straight coupling (you may need to remove the plastic covering if you use the end of your old liner), although I find that that some couplings tend to bite onto the tube and become difficult to remove them, but someone on here will be able to recommend a decent make, rather than a cheap one on ebay.