Can anybody advise me on where in the UK I can get a second assessment through the NHS.
The NAS advises:
"If you disagree with the outcome of the assessment, you may want to raise your concerns directly with the team of professionals who assessed you. You can consider requesting a second opinion by going to your GP if you have had an assessment through the NHS."
NAS - What happens during an autism assessment > Assessment results and report
This section of a related article also explains how you can escalate your complaint beyond your GP's practice and includes details of PALS (Patient Advice and Liaison Service), who might be able to advise and support you:
NAS - What can I do if formal support is not offered or is not enough > Health services