Hi Mark, I had to diet both of mine after coming back from holiday to find them at 6kgs each!!!! The vet said to go as gently as possible as fast weight loss can be risky.
You prob have to use diet food and weigh every few days. Once the weight is ok, you can move to senior. Mine ate a RC food for years and once I'd got the quantity right pd their weight was completely stable. Personally if you don't get too many complaints, reckon it would be easier to feed just one diet food rather than mix and match as they all have such diff calorific content that it may be difficult to work out what or how much to give! My vet said a 3 month diet to lose just over 1kg (they managed that, lol).