Ah. thanks Tony. So a blip during the EastEnders opening titles (particularly on a Monday or Friday, when it follows the news summary) would be the switch back from NET1 to a clean feed?
It's actually the opposite to that.
The glitch you've noticed on the EastEnders titles has absolutely nothing to do with the previous news summary.
EastEnders, like all network programmes, is joined (and left) using a clean feed (which is generated by a clean feed enabling flag being put on that event in the London schedule).
The glitch you sometimes notice is at the point the Nation switches from the clean feed, back to the sustaining network (dirty) feed. This is done so that the director/announcer in that Nation can get on with other tasks (writing, previewing, line-up, going to the loo, etc) without having to worry about monitoring the network programme constantly.
In ideal circumstances there shouldn't be a noticeable glitch. However the clean feed and dirty feed can drift apart, for various reasons... e.g. if there are any changes to the many bits of kit the circuit is travelling through, or if one feed is on the main circuit and the other is on the reserve.
EastEnders titles make this particularly noticeable due to the rotation and pull-out from the river. It can be minimised by waiting until the end of the titles. Glitches are less noticeable on a nice static shot.