Have a bit of an update on this:
Ended up going with 7" team dynamics pro-race 1.2, et 15. There was no way we were going to fit an 8" rim under standard or euro arches, plus the 8" rims are more pricey.
With -ve camber and the large body coilovers up front we still need to run 10mm spacers up front and 5mm spacers on the rear (plus a bit of rear arch persuasion inwards with a hammer and a bit of wood!). So et5 up front, et10 rear.
We first of all bought some Euro Raylle arches form Maximotorsport (https://www.maximotorsport.com), but after trial fitting them it was clear that they give only a very marginal increase in width over the standard arches (2mm ish), They are wider, but because they sit higher up the wing (therefore further in) the extra arch width doesn't equate to anything when fitted on the car.
In fact I managed to widen the standard arches more by 'massaging' the wings outwards, and using some spacers behind the arches.
But sadly not enough:
Tony from Maximotorsport was very helpful (despite Parcelforce's best efforts to cock things up), and swapped the euro arches with a set of his 'narrow' arches. These just cover the 205 width tyres , and only just, but I think they'll do the trick unless we get a super awkward scrutineer who gets the protractor out!
The front arches needed a bit of a trim as the tyres rubbed when the suspension was compressed - all sorted easily with an oscillating saw.
I also totally forgot that we'd have to remove the standard trim leaving rivet holes along the side of the car - quite happy with my cheap and cheerful solution to this problem. A £20 stripe sticker set for a Peugeot 308 of ebay; fits the 205 perfectly, and looks pretty good!:
Had a track day at brands on Monday to test the car before the new season - I cannot believe how much extra grip we had when the track dried out. The car felt fantastic and nicely balanced, it just went where you pointed it. Noticed most difference at Druids and Mclaren, far less understeer and way more traction allowing us to get on the power earlier. Can't wait for the first race of the season!
Here's the car on the grid for some promo shots for the series: