non start/imobiliser problem 200 tdi commercial?

Hi
My 94 200 tdi will not start on the key.
this is since the battery ran flat after being left for a while whilst some bodywork/welding was being done.

now when i turn the key all that happens is the red led light and indicators flash ?,if i short across the starter motor it starts straight away.

Im assured by the welder that the battery was disconnected whilst he was welding, but that it wouldnt start ever since he removed the battery to charge it.

Does anyone have a clue please,where do i start?
( i have no alarm key fob,but the alarm/imobiliser was working before just on the key)
thanks
Gareth

Did he fit the battery correctly? All wires positioned and contact made?

May sound silly and probebly is!

But I think im right in saying that if you connect the battery then lock n unlock the disco with the key in the door not the blipper-me-jig it resets the alarm/imobiliser ?!?

Give it a try if you havn't already - I may be wrong my mrs reminds me I normally am!