Declarations are for the installer, not the owner
A FENSA or Gas Safe certificate is issued by the competent person — the trade. If you are being asked to sign anything that says you accept the work as compliant, you are being asked to assume risk that is not yours.
Common framings to watch for
'Sign here so we can close out the job', 'just confirming the boiler is in', or a generic Completion document that mixes practical completion with compliance acceptance. Read every line.
What to do instead
Ask for the relevant trade certificate by name, issued under the competent-persons scheme. If it cannot be produced, the work may not be compliant at all.
Frequently asked
Your contract terms govern this, but it is a reasonable position to take in writing.