Who's to pay

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I have full British Gas cover. electrics, plumbing and drains since a month after my tenant moved in nov 2009. So they have serviced the boiler and carried out repairs over the time as needed. For some reason my tenant had a problem with the boiler calls out a local plumber who I previously used (before brit gas) of details she still had to the repair of £180. The first I hear of it is when the bill arrives from the plumbing company. I cannot understand why they did the work without querying with the landlord and also where I stand now as she states she cannot afford it. Also problem still not sorted and now had to call out brit gas anyway.She states she thought it was still the company we previously had despite not used for months! Any advice please on how to proceed.