Yes, my old provider was SSE.
I never said I intended to move VM. I am with VM who lost number during transition.
I do though intend to move to BT if I can get my number back this way.
All I can suggest is that you contact SSE, and see if you can return to them, and get your number back.
If not, all you can do is to tell your friends and family your new number.
So presumably you were a SSE customer, moved to Virgin and although VM now supply your service , either VM or SSE either never progressed or messed up the number port , leaving you with a 'new' number that VM allocated to you.
As already stated , if SSE 'own' a number range on your 'exchange' area, and the number in question is within that number range, then you would have to speak to SSE , and possibly consider moving back to SSE, if the number is that important to you, and obviously you should try and get some sort of 'guarantee' from SSE that if you re-join , they the number will be allocated, but , to be frank, that is not likely.
If the number was originally a BT number , that was at some point 'ported' to SSE ( and then lost as you moved to VM) then it's possible that if you became a BT customer ( with any old 'temporary' BT number ) then at some point you may be able to renumber whatever the 'temporary' BT number is, to the one you really want, but again , BT may make a best effort to do this , but probably wouldn't guarantee success.
If the SSE service you had was a WLR type service ( so although it was SSE that billed you, it was in effect a 'BT' service rebadged and sold to you by SSE,) then again the number would revert to the number range holder which may well be BT.
Ofcom have a website where you can look up number ranges and see which CP ( communications provider) has been allocated that number range, if the number is a SSE number, BT will not be able to get it for you even if you were willing to take BT service