I have done the following on a few occassions without any problems.

I removed the action from the stock for a thorough clean. After re-assembling the hammer now drops when closing the bolt

I remove the action from the stock and problem goes away!

Any suggestions on what might be causing the problem?

Some details;
Mauser M98 action
Timney sportsman trigger (fitted by me)
Boyds laminated stock (glass bedded by gunsmith)
These have worked flawlessy together for the past 3 years