@Alun135 firstly you do not have to organise anything regarding porting (keeping) your number when moving suppliers, this is all handled by your new supplier.
What you must NOT do is give notice to VM before the port has gone through, annoying though it is and may involve you paying for two suppliers at the same time, the OFCOM rules are that the service from the original supplier has to be live with no disconnection requests pending, for the port to work - so, for once, not VM’s fault.
So deal only with your new supplier, ask them to process the number port and see what happens. The onus is entirely on the new supplier to sort this NOT you, just make sure that you don’t contact VM in any way.
Now there are some circumstances which means that the number port can’t happen for technical reasons - it’s not usual but does happen from time to time