Thanks all for your various advice. I have the unit reassembled. As my milling machine is in Dublin I used my timber thicknesser, which I have here in France. I removed about 1.3mm and this gave just enough clearance. This means that compared to as assembled when new the amount of swelling that has occurred is nearly 4mm. I removed 1.25mm this I can say with a fair degree of accuracy as it was on a metal working mill. On reassembly I had to leave out the 1mm shims that were fitted when new. I now with the thicknesser removed about 1.3mm. This lot comes to over 3.5mm. When you look at the dimensions of the slider this represents a very large effect.
The only way I can suggest an explanation for this is as follows. The swelling is way beyond any figures given for the effects of water on nylon. The nylon must be expanded in a sort of a way beyond its elastic limit with water take up. It dries but does not dimensionally recover. It then takes up water and expands again. It is probably due to the structure of Nylon which I have seen described as partially crystalline.
As an anecdote I understand that Nylon shirts were killed off by the fact that in taking up perspiration they also absorbed body odours but these would not wash out resulting in clean but still smelly shirts after washing! These two issues might well be associated.
I will look at getting the four sliders made from an alternative material. I will with the various advice settle on the one that looks best. Thanks all again.