My question is how should an employer calculate a buy-out of existing terms & conditions of employment: Is it usually calculated on difference of annual salary between existing T&Cs and new T&Cs x number of years' service or is this calculation at the discretion of the company?

Unfortunately, there is no one-size-fits-all formula in such circumstances as much depends on the terms being changed, the reason for the proposed change(s) (for example if the proposals are to avoid redundancies, the changes may be seen by staff as the lesser of two evils), the bargaining position of the parties, whether unions are involved etc. We have found that many employers often prefer to offer non-financial benefits as an incentive for employees to agree to any proposed changes e.g. an extra day’s holiday entitlement.

I have to say if you are considering offering the financial difference between the existing terms and the proposed new terms, it begs the question as to what the