Hi Pete,
You're correct to observe that now is not the ideal time to judge an air conditioning system too critically. However, the most common reasons why your Rover is not as cool as your other car are condenser deterioration and/or slow leaks of refrigerant.
If you believe that this has only become apparent after the garage's work then ask them to check the ancillary belt tension and whether they discharged and recharged the air con. for the timing belt change (it's not necessary but two of the compressor mounting bolts do need to be removed).
Resist the temptation to buy a new compressor without a firm diagnosis. Old and high mileage does not necessarily equate with unreliability, in fact often it's quite the opposite!
From my journey in a brand new BMW 330 at the weekend I am happy to reassure you that its air conditioning was no better than the 23 years old system in my 75.
Simon