I hope that someone else comments on your query, Ladyessex, as I might well not be right.
It is the statement that you see 'bubbles rising to the surface' that catches my attention. These bubbles could well be Carbon Dioxide rising from decomposing vegetation (dead pond plant leaves or wind-blown tree leaves) accumulating on the bottom of the pond, decomposing and robbing the water of oxygen. Hence, dead fish.
Do you clean the pond out every couple of years or so? I don't advocate being too 'clean' with a pond but it does need some attention to support fish.

Within a period of two weeks one winter all of the three ponds that I had fish in were completely clean out. Over fifty fish were taken by mink and many of the fish were eight to twelve inches long. Nearby a pond retailer who sold koi lost thousands of dollars. The mink had taken every one of his koi stock and his ponds had netting which was supposed to keep such an occurence from happening. I am in the USA but I know that mink which had escaped from mink farms in the UK are now free breeding there so this might be the cause of your missing fish. Spawning fish chase each other around the pond nose to tail near the surface and you would have no trouble seeing that. The bubbles rising to the surface are just gasses produced in the bottom muck layer.

More likely to be heron thank mink to be honest...

I think that Bubaholic is on the right track. The bubbles coming up from the bottom would be the rubbish breaking down and giving off gas. If the fish were spawning then they would be very active and you would see them. I would therefor empty the pond and have a clean up before the plants started to grow and you could get to the bottom of the problem.

its a bit early and cold for them to be spawning just yet. You would certainly see then as the males chase the females and try to force them into corners/up to the surface. the females often breach and when they land back on the water the eggs are released. they certainly make a lot of splashing.

if they have died they usually float to the surface.
the bubbles do suggest CO2/CH4 release.

can you see any invertebrates in the pond like waterboatmen, gammarids or other such?
Do you have frogs visiting and spawing in the pond?

if the pond is netted then it is unlikely to have been cleared out by predators.

There are some water boatmen & gammarids in the pond, there was some frog spawn that have hatched but I cant see the tadpoles in the water now. My son in-law saw a fish yesterday but no sign today so far.
I was wondering if I should put some rain water in the pond from my water butts or would it be a good idea to pump some water out to see whats going on?

Personally I'd leave alone at the moment, pumping water out will disturb the invertebrates and, unless the pond is low there is no point in adding more water.

Agree with Moon growe. What is the diameter and depth of your pond?

And is it netted across the top to keep the likes of a heron out or just at the sides - which is what I read it as...

My pond is W240cm x L155 x D58....211galls/983Ltrs.
I have been busy, I got some sludge out with my big fish net, sorted out the filter & got it going & stared up the waterfall & fountain, water was pea green but has lightened up now, I put some clean rain water in too, I took a sample of the pond water to "Swallows Aquatics" all was ok, bought some aquatic plants for a ledge also some "GoGreen", I hadn't fed the fish if any for 3days but today I just put a large pinch in & a few gold fish came to the surface but when I moved they darted back down again, I hope I will see my two lovely Shubunkins as well soon.
I'm a Happy Bunny Now LOL.
Thank You All for your Help & Advice, this is truly a Great Site, you have helped me a lot now & in the past :o))
xx Jackie xx
I have it netted now LOL xx

Update on my Pond
I have added some pics at the top of my pond, Before & After.
My fish are now coming up & over to me to be fed, it's nice to see them with their mouths opening & shutting as to ask for the food LOL :o))