Aha.  Thanks folks - white sauce on top only, more pasta layers.  Will report back!  (Could be some time, I make a big 'un when I do one and we're still eating the current one.  The meat is venison mince with leftover home-cured gammon and it is absolutely scrumptious.  Just looks dreadful!)
And Simon O, over the years I have tried it both ways, pre-cooking the lasagne sheets and not.  This last one was not pre-cooking the sheets.