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I must be doing something incompletely because when I write off a sales a/c bad debt, I have to post the vat portion separately and after the event.
There must be a setting somewhere that debits the bad debt control a/c and the vat control a/c?

Are you on Standard VAT Accounts / Cash Accounting or a Flat rate scheme?

Assuming Standard Accounting
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What you describe is normal, by default, debts are not available for Bad Debt Relief, there are a bunch of criteria that need to be met.
http://www.hmrc.gov.uk/vat/managing/rec ... -debts.htm

You need to do a separate journal to reclaim Bad Debt Relief

The Journal is from your Bad Debt to Sales Tax (assuming Sales invoice) It is T9 on the Bad Debt and T1 on the Sales Tax account.