Hi Rojie,
Thanks for taking the time here!
I've found some good data on wheel-size.com and
It looks like my gen2 16" rims are 33mm offset (ET33) (I've yet to check but apparently the ET number (the offset) should be stamped on the rims somewhere) and as the 16s on gen 1 are 39mm it's not much different (6mm).
These wheels I'm looking at getting are ET45 and from the above website I noticed that standard 18" wheels on gen2s (with wider rims and tyres) also have an ET of 45. I've worked out that this puts the outer side of the wheel basically in the same place in relation to the outer bodywork (wheel arch) as the other standard wheels of different sizes (with the extra width of the tyre and wheel, all on the inside, under the arch).
So, I'm now thinking that an offset of 45mm is fine for gen2s in terms of wheel bearings and space under the arch (above and behind the wheel), if thats what bigger wheels run at.
The addition of a 5mm spacer should put the outer rim edge of the new wheels within a mm of where the current ones are, and reduce the offset accordingly. So that's good, the only thing I've got left to work out is, as Smurf pointed out, is there enough thread on the wheel bolts to safely accommodate a 5mm spacer (the people selling them seem to think so!)?
Anyhow, I'll ponder on that one a little longer and see what comes out! Cheers again for your help and sorry for the long-winded wheel-tech ramble I've put you through!
Oh and btw, that willtheyfit site was invaluable in my deliberations, thanks 👍
Simon.