Starting problems

Having changed the glow plugs last week the old girl is now struggling a little to start, she turns over on the starter and 'nearly' fires for about 30 seconds and then when she does start I have to keep my foot buried on the accelerator to keep it going at very low revs, the revs then build up really slowly almost as if its bogging down for about 10 seconds before suddenly rising normally and the engine then runs and drives without issue.

Would appreciate any thoughts to the cause

Did it start ok on old plugs? Are they wired in series or parallel?
Glow issue or fuel issue I reckon. I always start mine foot to the boards as per handbook. Adjusting tappets improved my cold starting and initial engine pick up from cold

Your not losing pressure out of the glow plug threads as low compression will make it hard to start.