Card payment processing fees — are they an allowable expense for my limited company?

Yes — card processing fees are fully allowable as a business expense, deductible against corporation tax as they're incurred wholly and exclusively for your trade. Record them as a separate bank/finance charge expense rather than netting them off sales, so your turnover figure stays accurate for VAT and your statutory accounts. If you're VAT registered, check whether your provider charges VAT on the fees: the monthly service element usually carries VAT, which you can then reclaim on your VAT return, while the interchange/merchant fee element typically doesn't. And at several hundred pounds a month, it's well worth reviewing your provider's pricing annually — these fees are negotiable far more often than people realise.