Dub1,
Going back the planning issue:
The plans attached to your contract are in a way less relevant than the plans on which planning permission was granted. It is a condition of your contract that the builder will have to provide you with a declaration of compliance with planning permission and building regulations on closing. This is a very important requirement, and in my opinion, inability to provide it would be a fundamental breach of the contract, sufficient to allow you to break the contract and get your money back.
If the property that was built is so very different to the one that the council gave permission for, then no engineer/architect will give a cert unless the builder goes back and gets retention permission.
I wouldn't put too much emphasis on the 200mm height difference - to me this doesn't seem to be too big a deal - it is a technical breach but doesn't seem sufficient to impact on your safe enjoyment of your property (engineers please correct me if I'm wrong!) - but the difference in plans is obviously very significant.
Call your engineer and ask his opinion. Would he give a cert of compliance for a house that is that different from the plans? If he says no, then call your solicitor and fill him in ..
And yes I do think you should stick to your current solicitor for now.