Help needed classic 06 plate rear diff lock is flashing when I try to engage it (I assume it should come on solid) 4x4 works ok is there a common fault before I start tracing vacume pipes etc Thanks tried 4x4 H and L standing still or just moving ..

Might want to speak to @Major Malfunction , he had issues with his difflock and ended up writing this How to remove a rear diff-with diff lock

Is it the diff lock light on the switch or the green light on the dash that flashes when you try and engage

If it is the diff switch flashing this is normal. A quick check is to pull the air pipe off the rear diff engage 4L with the engine running get a solid green ligh on the dash, then have someone feel for air being pushed out of the loose pipe, you should also hear a compressor buzzing which is located high up under the tub bolted to the chassis. If air is blowing out of the pipe pinch the end together to stop the air, you will now hear the compressor cut out, release the pipe and the compressor will start up again.
This test will tell you that the diff is being powered , the next test is to check the switch, replace the air pipe and then disconnect the switch on the axle, located on the O/S, then with an electrical tester check the switch by again engaging and disengaging the diff switch, or a simpler way is to put a length of pipe onto the axle and just blow down the pipe whilst testing the switch. This test will let you know that the mechanism in the diff is working, if you get no joy from this switch it does not necessarily mean the diff is not working it might be the switch is buggered, and to change the switch requires the diff to be removed. It's a very simple system and as long as air is blowing into the diff I would be pretty certain that the diff lock is ok.