Ongoing problem with British Gas Invoice

OK, let's hope this is in the right thread now as apparently I posted it in the wrong part of the Energy forum before.
I have had an issue with a British Gas invoice that has rumbled on since July and I'd really appreciate some advice on what more I can do.
Sorry this is long...
I have HomeCare 200 with British Gas to cover heating and plumbing, for which I pay a £50 excess for each problem. I called them out in July to fix an issue with a tap in the bathroom sink. After their plumber (sent from Dyno Rod) had left the house, I noticed the shower head had been taken from its cradle in the bath and was dripping (I hadn't done this). A bucket left in the bath was full of hot water, so the plumber had obviously done something with the taps in the bath. I also noticed that the tap on the sink was now loose – I went outside to speak to the plumber, who was still sitting in his van. He said he didn't have the right fitting for the loose tap in the sink, but it wouldn't be a problem. He didn't return to the house to look at the problem.
To cut a long story short, when I returned home from work later the same day I heard the sound of water running in the pipes in the bathroom, the hot water tap was hot to the touch, and hot water was dribbling constantly out of it.
British Gas were called again, a different plumber arrived, said the bath taps had been damaged and that I'd have to buy new ones. I did, at a cost of £90 to me. He also said the first plumber had fitted the wrong gland to the bathroom sink, hence the loose tap.
A week later I received two invoices for £50. I rang British Gas and they said it was a mistake; we shouldn't have been charged two lots of £50 as the second visit was a recall, not a new problem. I paid the one £50 only.
I received another invoice for £50 in August. I rang British Gas again and had the same answer. It was an error, I could ignore it, and no more would be sent.
Another invoice for £50 arrived in November. Same phone conversation, same assurance from British Gas, It was an error, ignore it, it has been amended on their system and I won't receive any more invoices.
Yesterday (12th) a red reminder arrived threatening legal action if I didn't pay £50 immediately. I've panicked and paid it, but I'm steaming and very upset about it.
Can I threaten British Gas with anything or do I just have to suck it up? I've now paid them £100; that with the £90 I had to pay for taps that weren't damaged before their plumber came here, means I'm out of pocket by £140.
Any advice would be gratefully received.