Hi Mike,
Might be time to get a Drainage Engineer involved, not worth scratching your head, guessing at a remedy, getting the work done then finding it hasn't solved the problem.

SSM

_________________
Measure twice, cut once!

Its definately helped reduce though not cure the damp. I am open to paying for advice but unsure who to approach... A drains type company or damp specialist? Have spoken to damp co in past who said similar level of damp in adjacent room was normal because it was below dpc. I suspect he didnt want the work. A damp proof injection is evident has been done in the past in both front rooms.

I generally dont have much luck in finding good tradesman and usually end up doing the work myself for that reason!

Just to update on this... I drilled large holes into the mortar from inside to see into the cavities, inside if which look dry. The damp has definately improved considerably since installing the waterbutt.

I plan on digging a 1.2m hole in the garden next to see if it fills up with water overnight to check ground water level.