Odd problem with Samsung Galaxy S Plus

I wonder if anyone here can explain something which is really puzzling me.

I have two phones
1) ZTE Skate (OMC) bought from Argos, unlocked and flashed with CyanogenMod 7.1
2) Samsung Galaxy S Plus (I9001) recently obtained from Virgin on a contract.

and 2 SIMs (let's call them 369 and 416), both from VirginMobile.

Until 2 weeks ago 416 was in the OMC on a Virgin data bundle and 369 was in an old
Nokia as my backup (PAYG).
Two weeks ago I got the SGS+ from Virgin and put 416 in it, and put 369 in the OMC.
Everything was working fine until I ordered a spare battery on eBay for the SGS+.
Seller claimed battery worked on i9000 and i9001.

When the battery came I charged it up (it came with a mains charger) and tried it
in the SGS+ but the phone reported "no service" even though I had a 4-bar signal.
Concluding that the battery was either defective or unsuitable I put the original
battery back and all was fine.

The next day I decided to try it again (no, I don't know why!) and the result was the
same as before, but this time when I put the original battery back in it was still "no service".
No matter what I do now the SGS+ does not receive service with 416.

Tried 369 SIM in SGS+ and it works OK.
SIM 416 works fine in the Skate (OMC).
And that's how I'm running now, but I want to use my data plan on 416 in my nice new SGS+.
Any suggestions?