You need some pcd adapters to convert your 5x100 hubs to 5x112. You can buy some from eBay like this:
http://cgi.ebay.co.uk/4-PCD-Wheels-...arts_Vehicles_CarParts_SM?hash=item5d27410307
This will mean your wheel's PCD will now effectively be ET27 all round. Whilst this would hopefully be acceptable at the rear (as the beam is narrower), the front wheels may well stick out too far. Plus it will do funny things to the tracking at the front, for reasons I can't remember!
So you're looking at over £100 + cost of wheels/tyres, and any associated archwork that may be needed at the front if they become too wide.
Another option is to redrill the wheels. This involves welding up the current holes and drilling new ones - expensive and inherently dangerous if done badly.
Perhaps look for the same/similar wheel in the correct PCD?