I have been told that the ILR lasts as long as the passport it is in and when the passport runs out, although you dont need to apply again you have to pay again to have it added in new passport. fact or fiction. many thanks

See also Visa For Uk In Expired Thai Passport, What shoud we do now? advice sought

Entry Clearance:

If a passenger is a returning resident who requires an endorsement in a new passport the Immigration Officer may only endorse that passport with an open date stamp on arrival. The passenger would then be required to apply for a UKRP or travel with their previous passport.

ILR is indefinite, even if the passport the stamp is in has expired the ILR is still valid.

If you do wish to transfer the stamp to a new passport then yes, you do have to pay. However, there is no need to do so and many ILR holders carry both their valid passport and the expired one which contains the ILR stamp.