Thank for response Oliver341. I'll try the DDNS route

Thanks for your response candlerb.
I will try the DDNS route.
If addresses changing is the norm, then that's fair enough. The reason I though they would be sticky is because there are at least two or three other installations that I look after....two VM and one TalkTalk, plus my home BB service (which is VM) all of which have had the same public IP address for yonks. Given that these are all in the same town, would the VM ones all terminate at the same VM BRAS?
Sorry to waffle on....I'm just trying to understand why it is inconsistent.

In reply to a post by RobAHorn:

there are at least two or three other installations that I look after....two VM and one TalkTalk, plus my home BB service (which is VM) all of which have had the same public IP address for yonks. Given that these are all in the same town, would the VM ones all terminate at the same VM BRAS?

Sorry, are we talking about Vodafone, or Virgin Media?

Are these all FTTC circuits, or FTTP, or some mixture?

You'd want to check whether the VDSL link stays up (the PPPoE session is terminated), or if the VDSL link itself goes down, which might be possible to see in router logs.

FTTC lines are likely to be interrupted for random reasons, including someone turning on their TV. They will be on different cabinets or different line cards in the same cabinet, and will be on different copper pairs, so basically nothing can be inferred about the reliability of one circuit from the others. In any case, an FTTC line which only reconnects twice a month is pretty good.

Such problems are very unlikely with FTTP. Light is light, and immune to EMI.

Sorry to confuse matters! The service in question at the start of the thread is VF, FTTP. The others I threw in as comparisons, one is FTTC and the rest are FTTP. The VF one is the one experiencing some sort of interrupt but I have a VM one that seems to be having similar issues. The comparisons were simply to demonstrate that I have two VM installations that seem to retain the same address and one that does not, which doesn't seem to make sense in the light of your explanation that BRASes rebooting would normally result in a different address being served to my router.
Needless to say I am struggling to get VM support to take an interest, but I admit I haven't engaged VF support yet
Anyway, I appreciate your responses....all good learning !