Kevin, having just got hold of a SLEC fittings kit for a birthday present from my dear wife I can confirm that in fact they are not in plastic, but white metal. All of them. There are no instructions or parts list. They are the old Yeoman fittings kit, a set of which I was promised by Ivan of IP Engineering as I was involved with his works for a while as a pattern maker and he showed me all the original patterns for what I assume is what SLEC now sell, as Ivan flogged off all his Vintage Model Boat Company rights to them. Chances are he is now just casting fittings sets to SLEC as they don't have casting facilities at their place in Watton.
I had an original Mersey Marine set with my Crash Tender for my 11th Christmas, but it was stolen and they no longer exist alas.
Kevin, just checked the Model Slipway set and assume they are the very same as the SLEC set as the items appear to be the same. I'm not sure if I got net rail blocks with mine and can't check until 22/7 as my wife won't let me have the set till my birthday! But if that's the case you are just paying a fiver more for the same set. I'd say get em from SLEC! Cheers, Martin