I bake around 8 cakes twice a week. These are large celebration cakes in a professional oven. These have been made the same way for many years and occasionally the bake would be patchy as in the picture, but lately this has been happening a lot more often, maybe every 2 to 3 bakes. We have tried different people baking the cakes, different flour and different butter / margarine but so far none of these seem to have made any difference. Please can anyone tell me what causes the dense patches in cakes like the one below.

Any changes to your baking powder or eggs? Also, have you had your oven thermostat checked lately?

Hi, there have been no changes to the eggs or baking powder. The oven does get serviced, and often it's not the whole bake thats effected. It is also cakes from different parts of the oven each time so I didn't think it was the oven. It does usually have this pattern to it though, or mainly in the centre. I have tried mixing the cakes more and less, but neither of these seem to have any impact either.

Hmmmmm, that's a tricky one. And you haven't changed your recipes at all. I hope someone else can give you some insights!