singaporegirl,
My understanding of the rules was that to be covered by the Consumer Credit Act as stated, you had to spend £100 or more on the credit card to be covered and from personal experience that does not guarantee a bank is going to roll over backwards and just pay up in the event of a claim without you putting up a fight. I cannot see how by part paying cash you have entered a credit agreement for the cash portion.
IMHO if someone is thinking about doing it this way take professional advice from CAB or Trading Standards first.