I think the clue lies in the lack of response to the control lever. Your engine's governor is jammed due to condensation and rust over the winter. I regret to say that, although I have heard of this problem with the Bukh DV20, I have not experienced it myself. Try Bukh Diesel (UK) on 01202 668840. They are very helpful with advice over the phone and can supply parts by return.
Neil
Neil Sinclair
Neil Sinclair
Seal 28/27
'Andiamo of Exe'