Around midnight I received a text from Virgin (from the same number that sends the 'We've refreshed your allowances' texts). This one though just said YGIB: D TAG followed by a long number.
I have recently upgraded my phone and moved from a regular contract to a Freestyle one. I don't have insurance and don't have Mobile Rescue installed on the new phone, though I did on the old one. The new phone is working normally, I don't have the old one any longer so can't say about that one.
I take it you didn't search the forum before posting your question?
The text is an initialisation message for Mobile Rescue. It would be "intercepted" by that app and used for its setup, so you'd never see it, if Mobile Rescue were installed. Because you don't have it installed, it is presented to you by your default messaging app.