Oh dear.
No, he can't visit or have viewings whenever he wants to, it has to be with your approval (even if it says otherwise in your AST). You have the right to 'peaceful enjoyment' of the property, which means he cant come round bothering you.
You can only have one rent increase in a 12 month period, so it would depend on what rent you agreed at the start of any new AST.
No, you can't legally withold rent unless you come to an agreement with the landlord first.
Just as an aside, is he holding your deposit within one of the tenancy deposit schemes? You must have signed up around the changeover time to that scheme?
Personally, I'd start looking for a new place to rent, one where there's no building work going on and the landlord knows what he or she is doing!