Cheers for the reply DaveRo
You might be right... maybe the SIM and foreign IMEI are linked, and once the SIM dies after not being topped up after 6 months, then it is difficult to get the foreign IMEI back on the grid again...
I haven't had time (or inclination!) to chase it up further, but thought I would raise the issue here on the forum in case anyone else has the issue or finds a solution.
Let us know how you get on with moving your SIM to your new phone... or maybe it might be easier just getting a new SIM registered to your new phone...