Defender 300tdi, drive problems ?

Hi guys my defender is in the garage right now, but I was just wanting to see if I can get some ideas of what the problem could be from Landyzone.
Ok well it's driveing ok as in starts, and gears seem fine, pulls of ok, 1st 2nd 3rd and so on, but if iv had to slow down before an incline it's not giving me the full drive as it did. And it's finding it hard to get up. I can press the throttle, the engine rev's higher but it's not giveing all the power for the rev's.
It's with the local garage guy that owns his own defender, he said could hear from the sound that the air intake (snorkel) was making he was thinking one of the cylinders wasent firing "tappet's".
What do you think ?, I'd be grateful for any ideas.
Pete

I think he's suggesting a valve being kept open and compression is going up your intake (snorkel).

Yeh sounds a bit like what the guy from the garage said, and one of the cylinders isent firing. He also mentioned the clutch could be slipping, and to be honest that's the best way of discribeing the feel of loss of drive, it's like iv got the clutch half in and it's not delivering all its power. So it looks like I'm going to be without my wagon untill next week