On reading installation instructions: item 2 reads, avoid mounting bell push on UPVC door frame, bell push should not be mounted on UPVC door frame as operating range will be reduced. Didn`t know about this limitation until opened packaging.
If I insulate bell  push with plastic or wood fillet  when fixing to door frame would this eliminate or reduce possible loss of operating range ie signal strength.
Advice please

Try it with the bell push held to the fram with tape before you drill any holes. Might work if the reciever unit is just going indoors by the door.

I used a Byron wireless door bell because I was concerned the metal foil covered insulation in my walls would cause problems with the signal. Our mobile signal is certainly affected.

The set up I have uses an ordinary wired push button on the outside with a short wire through the wall to one of these "wired push converter" on the inside. Basically it's one of their wireless door pushes without a button on it....

http://www.chbyron.com/Byron/Wiredpushconverter/68/68/90/45/model/p/294/SX32/

That can be used with any of their SX series of receivers/bell boxes. We opted for a battery powered unit that and can be moved around the house. They also have mains plug in units.

http://www.chbyron.com/Byron/DoorChimes/68/68/90/45/model

You might have real problems fishing for the cable. Why not use a cordless bell?

Mine is located on my PVC door and the receiver is two rooms away and I have no problem.