Do you mean cut the number of hours of overtime or the rate of pay? If you mean the former, the employer is under no obligation to find overtime for you, and can no doubt make a good business case for paying someone at a lower rate, subject to any sex discrimination issues
Buildersmate will know whether they need to consult/give notice of a change to overtime rates. I'm not sure they do- it may be different to just changing your rate of pay
Yes, it would be more cost effective foe the employer to take on another person than cpontinue to pay overtime at a higher rate. Miostly overtime is used to cover sickness, emergencies or extra busy times. Once it becomes more regular they are likely to create a new more junior position to cover this extra regular work. Overtime is very rarely guarenteed on either side so can be taken away in just the same way as you can in theory refuse it.