Preparing a VAT return

Materiality - do you need to chase for every £5 flat white receipt?
Previous treatment - you’ve got the invoices for last quarter, but maybe the person with the login is on holiday this month, can you assume the same treatment and chase them later?
Ask for exactly what you need - rather than sharing a list of all the outstanding transaction, apply the above and narrow it down to chase for the important documents, with specific mention to the deadline and consequence to make it clear why you need them.

Revenue if you’re not posting revenue through invoices, but journalling it into Xero, check the right VAT codes have been added to the journals
Prepayments/Accruals - VAT rates should be applied to the invoices, not the journals

Reviewing a VAT return

Revenue (Box 6) vs P&L - ensuring all sales accounted for in the VAT returnif you’re supplying B2B services outside the UK, these aren’t “No VAT” but should be “Zero Rated” to be captured in Box 6
Total payable/receivable vs BS - allowing for the timing of any payments (particularly for anyone on the monthly scheme) check your VAT account balance matches the return
Compare to previous returns - is the amount payable/receivable significantly different than prior quarters? If so, why? It could be legit, but you should know what has changed.
Consistency - check for any suppliers or customers appearing across multiple VAT codes, is this correct? Amazon-type business, this is quite likely, but perhaps not so much for software providers or contractors etc.

Check the larger expenses - every company is different, and the definition of “large” will vary. But based on your return, check the big ticket items have been correctly categorised and have the right documents.