Supplying certificate of compliance with council regulations when job is finished

They are not exactly obligated to do it. But, it is in their interest to do it. Ask them what they intend to do.

I would feel duty bound to provide Building regs complience certificate,althogh it would be home owners responsibility ultimately.

The Building Control Certificate is issued by your Local Authority Building Control not the builder. Without knowing the details of your conversion it is difficult to determine if it would be subject to Building Regulations but I would be surprised if it wasn't. Your Builder does have a legal obligation to notify Building Control if they are undertaking works subject to the Building Regulations irrespective of if they priced for it or not. If you have any doubt you can contact Building Control direct and they will advise you and it is possible to get a retrospective Approval Certificate but they will charge you for this and will come and inspect the works and will want drawings submitted as if you were making an application from the outset. I would recommend you speak to Building Control in the first instance.

Almost all work building work (apart from very minor work) has to be overseen by your local authority building control or a qualified surveyor. If you don't have a "completion certificate" when you come to sell the property it will be a problem. The first thing that will happen is the buyers surveyor will want to know if the work was done under building regs and unless you have a "completion certificate" It will be a nightmare to do it retrospectively for instance how will building control know what insulation was put in the walls once they are plastered and the flooring is laid. Building control will not take the builders word that it was done properly. The whole point of building regs, is to provide the homeowner with proof that the work has been done to the required standard and for a prospective buyer to know that the house they are purchasing has had the work done properly.