Had fibre installed last month and OR supplied the modem. For £5.99 IMO it's a good idea to get PN's router even if you have your own. Then if(when?) something goes wrong you can swap to their router which PN's support people understand.
The case for having their router isn't as strong as having their modem/router for ADSL With fibre you can always ultimately connect a PC directly to the modem. If/when PN start to supply an integrated fibre modem/router then the reasoning will be the same as for ADSL.