5I'm currently cruising the K&A when without warning or any noise I have lost reverse gear.

I have searched technical manuals and have deduced that it is a conical clutch system.

I have disconnected my drive cable and tested it for movement. The cable operates freely.

I carried out the following check with the cable disconnected.

With the engine running I operated the gearbox lever by hand. It engages fwd drive ok, however on full movement into reverse the propshaft is barely turning. 

Is there any adjustment I can carry out which would enable reverse to engage correctly without affecting fwd drive?

Any advice and help would be grately received

It is a crap gearbox.  I changed mine for a PRM150; a world of difference!
Sorry, that doesn’t help your situation much.

Gearbox duff reverse cone clutch, your tests are correct and prove it. You checked the oil?

Better to bite the bullet and replace with a PRM, preferably a 150 hydraulic, then you need an oil cooler plumbed into the cooling system, but a PRM120 mechanical will do you well.