I would simply put something along the lines of 'The Supply of Goods & Services Act 1982 requires our item to be fitted to a proper standard of workmanship and to last a reasonable length of time. I consider in this instance that this has not been the case and would appreciate your rectification of the matter by means of a repair satisfactory to me by ...( insert date).''
I often get quotes for, order and sort out issues for several of the family businesses where the problem is not too complex and the 1982 act seems to be fairly straightforward saying this about 'reasonable length of time'.
What is reasonable?
It can be difficult to know what is deemed as being 'reasonable'. Here are some ways you can find out.
To get guidance on reasonable standards of work, contact a trade or professional association. If goods are being installed as part of the service, you could contact the manufacturers.
You could also ask an independent expert for an opinion on the work, or to test a repair. But be aware that this can be expensive and you may not always get your money back from the trader, even if the expert supports your case.
To get guidance on how long a job should take, ask other traders how long they would take to do the work.
If you haven't agreed a price and the trader appears to be charging too much, ask other traders how much they would charge to do the same job.
With regard to any goods which are supplied at the same time, any goods or parts fitted as part of the service or work contract must be:
as described
of satisfactory quality
fit for their purpose.