I don't know, of course, if a product we swear by here in the western U.S., is available in the U.K., but check to see if Lexol can be purchased. Best by far of any product I've tried. Get yourself a couple of cheap toothbrushes to clean the crevasses and the rough underrside of the various parts of the saddle such as where the jockey covers the skirts... You can undo the thongs that hold the conchos that hold the rear jockey to the skirt and that'll make the job easier. If your saddle happens to have one of the "padded" seats, don't be to liberal with the saddle cleaner/conditioner. That kind of seat tends to soak up any oil and immediately deposit same on your backside.
We have saddles here on a ranch that have been in service for 50 years or more and are still going strong as long as they're well looked after...
Additionally, it's very easy to take the stirrup straps off and clean and oil them. You might find that some of the strapping used to hold the stirrup strap rings on the saddle may need replacing.... as well as the rings for the cinches (if it's a standard double rig there'll be two of them... the rear one being leather only).