If we have edge protection of 3 sides of a roof (except ridge) and our roofers are using fall arrest blocks and shock absorbing lanyards - do we need a rescue plan?

Yes. Suspension trauma is a significant issue. I'm a bit confused as to your reference to a "ridge" though - is this a sloping roof then? This all assumes the roof is sufficiently high to allow the lanyards to deploy - restraint is often a more valid approach, but not without attendant problems.

This is true - considering other methods of carrying out the work safely at height first would be a more appropriate option rather than going straight in with fall arrest as your first option If you are going down this route after looking at your other options, then a rescue plan should be in place as a precaution. A multitude of scenarios could arise and if no rescue plan in place, it will create a very dangerous situation where time is of the essense From HSE site - hope its useful; Yes. The law requires that when you work at height, you also plan for emergencies and rescue. These arrangements should not rely on the fire brigade as this may result in a delay which may be critical. Rescue kits are available and suppliers can provide training in their use so that in house equipment and expertise can be provided.