Bongo Fury

From being no problem at all, our diesel bongo (Freda) developed a problem starting on the first start of the day. At first we thought it was possibly due to different fuel, as we were camping on the continent at the time but now wonder if it may be down to glow plugs. Basically, it used to start first turn of the key, then it was a laboured start (10 seconds or so and using the accelerator) whilst it emitted smoke from the exhaust and once warm it was fine for the rest of the day. Now, the battery cranks powerfully but it just won't start.
It has had several fills of fuel and long journeys since the issue began less than a week ago. The dashboard light for the glow plugs comes on and goes off as normal.
Any ideas?

It sounds like glow plugs, although it should still start eventually . Have you tried leaving it for 5-10 seconds after the glow plug light goes out?

Yes, but it doesn't seem to make any difference. Are they easy enough to change (I have a little bit of mechanical ability from off road motorcycling etc. but petrol rather than diesel)?
Are they effectively screw out/in like a spark plug or are there technical do's & don'ts.

How would I know if it was the relay as well as/instead of the plugs?

Thanks.

If you have a multi-meter then set it to 20volt range and clip between chassis and the glow plug rail, when you turn on the glow plugs you should see the rail go to 12 volts.
If it doesn't first check the fuse after that look towards the relay B2-04 top left below;