engine clatter

Hi people I am posting this for a friend of mine in the hope that
someone can help.His car is a 2007 focus 1.8tdci with 80k on the clock
(ex lex lease car with full service history) the problem is the car
developed a loud clatter from the top end I have heard it and it is
loud its coming from the front half of the engine Strange but you can
get rid of the noise by holding car at about 2000rpm  but outside that
it comes back in again, There is a slight misfire  at about 12-1500rpm
with no load on the engine and he tells me that the car is down on
power( I suspect the two are related)
The car is sitting in a garage at the moment waiting for him to make a
decision on what to do
The garage I think is stumped as to what it is  but have said it could
be:-
Little end bearing ( but why would that be intermittent)
broken valve spring ( possibly but again why intermittent)
bend valve ( car hasnt been overrevved why would valve bend)

They have also dimissed faulty injector "as they dont sound like that"
and have dismissed faulty hydraulic tappet (which was my prime
suspect) as its got shimmed followers
 Wha I can say is that the clatter sounds to me like its timed the
same as the cam and is harsh  and there are no fault lights  and oil
pressure light goes out instantly


Has any body had a similar experience with this engine or any good
advice to give  before we get into large expense  as the garage
suggestion is to put in secondhand engine

Many thanks
Alan