My second year with all mesh floors, and have noticed lots (50/150) of dead bees discarded on the mesh floors, I cant remember seeing this last year.
Is this related to especial cold weather, making it to cold for the cluster to remove, or is this the norm with mesh floors?

I've been using mesh floors for a while, and often see bees accumulate on them, especially so this year, with the very cold weather meaning the bees haven't had as many warm days to do the housekeeping. Since the hive will be colder just above the mesh floor, they're also less likely to go down to clear out dead bees.

The thing to watch is for the bees doing a good clean-out once the weather does get warm enough... if they don't do their housework properly, this might indicate a weakened colony that needs more attention!

thanks for that, I've since been in to check sugar bags, and they are all quite warm, so hopefully all is well and queens are laying.

With the warmer weather over the past few days I decided to clear my mesh floor hive yesterday and found around 500 dead bees right at the back of the hive on the OMF. Is this number normal? I've watched the colony and they seem quite strong, the brood box was quite heavy and when ever theres sun the bees are out in numbers gathering pollen. I checked through the dead bees and found no sign of disease.

While adding drone comb to 2 selected hives, not for varroa but to attempt to have drones flying in may, I noted that both queens (last years) were laying reasonable sized patches of eggs and brood at various stages. These were hives previously with a lot of dead bees on the OMF's, since the weather improved last week they do seem better at cleaning up.
It was suggested to me that I should leave the sticky floors in place during the cold to help maintain the heat, I've never done this, but have always tried to ensure that the there were no leaks around the top of the hive prevent draughts.
2 hives that are double Smiths brood boxes had high varroa counts at 3 and 4 per day, the singles are showing zero or just the odd bug.
More questions than answers as usual

As a final comment after checking my 6 hives all have survived with no damp mouldy combs and plenty of bees to kick start the year. All had last years queens although the quality was variable and 1 had become a drone layer and was quickly joined with another.
In our cold climate it does become more difficult to sneak a look withouth chilling the entire hive as the warm air rises dragging the cold with it as soon as the top board is moved, but this apart, I think the OMF's are better.