Would you like to PM additional details? I don't know exactly where I am going wrong, pattern seems to fit so far...
There's a 7 minute interval but every few hours the minute increases by 1 and then carries on the 7 minute interval so it can go like this:
12:42
12:49
12:56
13:04
13:11
13:18
All the data we have pretty much confirms that definitively.