Bit more research. This link

https://superuser.com/questions/6517.../651807#651807

Seems to say that blue + blue/white are the ones as these end up as pins 3 and 4 in the RJ11 and these are the pins the phones use.

I found this filter

https://www.onbuy.com/gb/network-cab...IaAr3bEALw_wcB

Which I could use by plugging into the main line patch and then connecting the filtered side to the phone parallel patches and the unfiltered to the modem (RJ11 modem lead into the rj45 ) not ideal (and expensive) but might work.