On the 26th September I went and bought a few bits in Lidl after work, the money has only left my account today. 
Luckily it was only £10 odd worth of stuff, but if had been a big shop and over £50 then it would have eaten up the money that is left in that particular account today and then forced me into an unauthorised overdraft.
AIBU to think it should not take 15 days for a transaction to process? I have never had this when paying by debit card elsewhere.