1st of all.. changing the width of the tyre MIGHT not make any difference, it all depends on how wide the alloy is.. 7J.. 8J 8.5J???.. IF the wheels are too wide it might not make an ounce of difference putting a narrower tyre on as the sidewall of the tyre will still be in the exact same place, as it has to fill out the width of the wheel.
If the wheels are not too big then as Charlie (Karma Supra) has said, it will be down to the offset of the wheel... the offset determines how far in/out the wheel sits in relation to the struts/arches, If the offset is wrong it is possible that spacers may help to get the wheel away from the strut ( I will give my opinion on spacers later)
As for the tyres hitting on the plate for the coil spring, then a lower profile tyre will help, but.........
QUOTE
The tiers on it at the moment are 225, 45, R17
It looks like is i put a 35 on the rim it will probably fit, as that gives me a extra 10mm to play
.. it doesn't work like that. The profile number on the tyre is an aspect ratio, a percentage of the given width, in the case above the hieght of the tyres you have on the wheels (225 x 45 %) = 101.25mm while a 225 x 35 % = 78.75mm. Work out how much lower (in MM's) you need to go then find the tyre size that fits what you need. (215x40/17 will give you 86mm in hieght, a difference of 15mm, and will be cheaper than 35 series tyres.
Now the spacers.. My opinion is this.. if the wheels don't fit without spacers they shouldn't be on the car in the 1st place.. with say a 5mm spacer on the car, the wheel won't sit on the hub properly and the full weight of the car will be on the wheel nuts/studs, not recommended. However, if you were to spend a bit of cash you could get yourself hub-centric spacers.. these bolt on to the hubs in the same way as the wheel does, they have holes in them that allow you to mount the alloys to, aswell as having a lip that the wheel sits on for support so that the wheel nuts/studs are not under any undue stress.
You just might find that it would be cheaper in the long run to sell the wheels on and buy wheels that actually fit the car.