Consevatory Overrun, advice needed please.

[Lonnie]

We engaged a local firm to put a Conservatory on the back of our Bungalow.

Start date 13th August, end date 7th September.

Unfortunately, in our naivety, we signed the contract, which says they cannot be held responsible for delays , on ours, or other jobs they have,
Twice now, a week at a time, no-ones turned up, Their fitter is due to go on holiday, so more delays.

Thing is, its causing us extra expense, and inconveniance.
This now the sixth week after the finish date, and looks like it'll take at least another two, possibly more.

Question,
does anyone have an idea, legally, as to what might constitute an 'unreasonable overrun'?.

Thanks.