There may not be a completion certificate, this was the state in 2012 which is after the work was done, I don't think the situation has changed since then.
'At present completion certificates are only required to be issued where the building is in scope of the Regulatory Reform (Fire Safety) Order 2005 (ie non-domestic buildings and blocks of flats) or, in respect of work on houses, where the applicant has requested a completion certificate at the time of submitting full plans. Where work is carried out on a house under a building notice or where a certificate was not requested at the time of submission of full plans there is no requirement for the local authority to issue a completion certificate.'
https://assets.publishing.service.g...uploads/attachment_data/file/8379/2076689.pdf .
It's likely that knocking a wall down was done under a building notice without full plans, the BCO would just inspect before the structural work was obscured, a search on building control on the LA site should show if full plans were submitted.
An indemnity insurance for this level of work is common.
If it was not a supporting wall building control would only be interested if any window was also bricked up.
I'd tend to leave it to the purchasers to decide what to do.
I agree...theywill have to sort and come to a decision not withstanding considerable evidence that the build alone has come cheap to them in a significant part of CheshireIn 1983 and 1985 I did substantial DIY building work on my house all with full plans and Inspection, the LA at that time did not issue completion certificates.
When the house has to be sold, most likely after my death, a solicitor is likely to ask for the certificates , my three children will be handling the sale, one of them is a retired chartered accountant and one is a chartered surveyor.
I expect the answer to many queries will be 'unknown', I doubt if any copies of the drawings in my possession will be handed over, they can be inspected at the LA, plans of that age are not scanned and on the web site.
No point in giving an argumentative buyer the chance to apply current regulations.