Hi Bob Ball,
You can pay in more than the £27750 but no relief would be given.
You have an annual allowance of £60k to limit pension contributions and it is the unused amounts from this that can be carried forward.
If you pay in more than the 60k limit you can bring forward the unused relief so that you do not have a charge to pay.
If you don’t have unused relief then yes you need to complete a tax return.
Please see guidance here:
Working out and paying the annual allowance charge
Thank you.