Thanks again for your reply.
Get your tape measure out and draw up some plans showing the changes to the property. (Obviously, if you've still got a copy of your instructions to the builder you can use those). Make sure that you know the types of materials used, including for the doors (as they could affect the fire resistance sections of the Building Regulations). Take some photographs and print them out.
Print out this form:
https://www.horsham.gov.uk/__data/assets/pdf_file/0009/26793/Regularisation-application-form.pdf
Send it to the address at the bottom of the form, accompanied by the plans, photographs and full details of the work (including the materials used and a schedule of the costs).
Once your application has been received you'll be advised of the relevant fee (which you can then pay by phone or online). However you can see what it should be, based upon the cost of the work, by consulting the right-hand column of Sections 17 onwards here:
https://www.horsham.gov.uk/__data/assets/pdf_file/0009/28557/L17_BC04-2017-Fee-Schedule.pdf
Then write a letter, addressed to
Leasehold Services
Crawley Homes
Town Hall
The Boulevard
Crawley
RH10 1UZ
Here's a rough draft of that letter:
"Dear Sir/Madam
In [month, year] my husband and I purchased of the leasehold of our home, at the above address, which we are now seeking to sell.
However the solicitor acting for a potential purchaser has advised us that, under the terms of our leasehold agreement, we should have sought the permission of Crawley Borough Council (as the freeholder) prior to installing a new stud wall, with associated doors, in [month, year]. We are therefore now seeking retrospective permission for this alteration to the property.
We enclose full details of the work which was carried out and we would be most grateful if you could advise us of the best way forwards from here.
Please note that a separate application for regularisation of the work, under the Building Regulations 2010, has been submitted to Sussex Building Control.
Further, an application for a review of the Council Tax banding of the property has also been submitted to the Valuation Office Agency.
Yours faithfully"
(Once again, send as much information as possible about the work with your letter, including plans, photographs and details of the materials used).
Then write another letter, addressed to
Listing Officer
Valuation Office Agency
Westmoreland House
Strand Parade
Worthing
BN12 6EA
Here's a rough draft again:
"Dear Sir / Madam
My husband and I own the leasehold on the above property, which we are now seeking to sell. However the solicitor acting for a potential purchaser has suggested that we should have applied for a reassessment of the Council Tax valuation when we divided a bedroom into two (through inserting a stud wall, with associated doors) in [month, year].
We are therefore now seeking a new valuation and enclose plans and photographs of the alteration to assist in this.
Yours faithfully"
(You probably only need to enclose the plans and photographs with that letter, as the costs of the building work aren't likely to be relevant. However it wouldn't do any harm to enclose the schedule of costs if you're in any doubt).
You might also wish to consider sending copies of everything to the buyer's solicitor, to show that you're determined to address his concerns as quickly as possible, but your own solicitor would probably be best placed to advise you as whether that's a good course of action or not.
[Disclaimer: I'm neither a solicitor nor an expert on Building Control matters but I hope that, by pointing you towards the relevant officials, I'll have got you started in your quest to resolve the awkward situation which you now find yourself in].