Cheers @pint - I enjoyed your cheque story! What a crazy world...
I finally made my decision and have just placed an order to switch to Zen for BB and VoIP later this month. The new router will be compatible with my current DECT handsets and Zen will port my existing number with One Touch Switch.
I'm expecting some, hopefully brief, outages for internet and phone during the switchover but can live with that.
The price I'm being offered by Zen is almost identical to my current PN package for FTTC plus phoneline. The exit fees are also less than I had feared, so it was a fairly easy decision in the end.
Will update as things tick along - the switch is booked for about 3 weeks time.
Had PN offered me VoIP I would have stayed put but didn't want to swap great customer service with PN for, err, posting cheques for £0.00 to EE!
Many thanks for all the advice on this forum, it's been very helpful.