Just in case you didn't know of this site, Shelley...........
http://www.planningportal.gov.uk/permission/
Click on "interactive house" to confirm your permitted development.
PD is exactly what it says ........ "permitted". You really need to do nothing other than to declare (pre-contract enquiries) the work you have had done, and, just state that it was under PD. You are under no obligation to "prove" this. It's simply a fact.
The trouble is, nervous solicitors for the other side often try to push their luck. At the last resort, but this is your decision entirely, you can apply for a "Certificate of Lawfulness". It's largely a money-making scheme for Planners. It costs almost as much as Planning Consent, and all it does is to confirm your work as legit. It does keep the lawyers happy though :o)
Assuming your house is not Listed, I don't think, from what you've said, that you have anything to worry about.
MUCH more importantly............ have Building Control been involved? (Building Regulations etc)