I think you should be able to exchange the 13kg one for a 3.9/4.5, rather than paying the "hire charge" on a new bottle. You may find adverts for empty cylinders for about £5 or even free, or you could of course post a "wanted" advert on this site's "Free Adverts" section.
Hi folks, managed to exchange my 13 kg propane bottle today for two 3.9kg ones.
They fit lovely in the locker.... Fantastic.
Thanks everyone for your advice