I know I'm weird but....

Why is it so expletive deleted ing difficult to find a set of 18 inch wheels without having to spend the budget deficit of America on a set of forged Rays or alternatively find a set of equally horrifically priced adapters to then have to reverse engineer the off set resulting in more emotional damage than that Steven He bloke off the Internet just to fit something eye burningly ugly off a defender or pick up. And no I don't see why I should have to settle for 16s just because that's all that exists. Sorry first world problem, but still. 

Supply and demand.

I had 18" wheels on my T5. Never again! So happy to get it back on 16. Cheaper rubber, longer lasting, better driving.
Smaller wheels are just better. Sorry I was fully aware you did not want to hear this

However it seems that sometime in the future I must change to too large wheels myself, 15" is a dying dimension with less tyres to choose from compared to 16" now.

I trust these 18” wheels are for low profile racing slicks otherwise a serious amount of trimming will be in order?

235.40r18 improve turn in and mechanical grip at the expense of comfort. Coupled with shorter springs and even more gucci dampers than it has now and away we go!

Two suggestions:-
Farmers Tyre Ltd, leamington Spa. They used to weld up new steel wheels made up with your PCD and a suitable rim, or ask at the dealer who will supply the tyres.

Roger