I recently joined LA Fitness on an 18-month contract and added my girlfriend as an additional member on a rolling monthly contract for £33 a month. Shortly afterwards, the gym emailed me to say membership costs were increasing and gave me the option of going on contract for a discounted rate. I assumed this was a mistake as I was already in a contract; I didn't notice my partner's details in the top corner – it was actually about her membership.

I rang a few days later to cancel her membership for lack of use, at which point LA Fitness remarked that my final payment, for December, would be £37. When I queried this, their response was basically "tough".

Although I have only lost £4, it is illuminating to realise that the website conditions for adding a member have remained at £33 per month. Nowhere did it say that the lower rate is only for a limited period. It is clear to me that this is a marketing ploy to get extra members on the books, and then hit them with higher charges later. AB, Woking, Surrey

We get numerous complaints from gym members about the complexity of contracts, despite tougher requirements for clarity from the Office of Fair Trading.

We contacted LA Fitness, which said that this shouldn't have happened and was a mistake. It insisted it would not have included a separate price rise for a partner, and in particular – in your case – one who had been added just two months before. The company hopes this is a "one-off" and has explained to you what happened, apologised and reimbursed you.

Encouragingly, it has agreed to look at add-ons more closely in future. It said: "When a price rise happens it's sent out across the members where applicable, however this one slipped through the net. In general terms those with contracts who have add-ons are considered and treated in the same way with the same offer/rise (or not as the case may be)."

You were right to point out that although this involved a small amount of money it will add up over the months and – with almost nine million people in the UK belonging to a gym or health club – it is the principle that counts.

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