Growths on Greengage leaves

sjj 919

Can anybody help identify what these lumps are (on around 40% of leaves on both of our greengage trees)?

I'd like to know if there is anything that should be done about them & whether I should be worried.

The trees are dripping in fruit & the fruit seems totally fine & healthy for this time of year / stage in it's growth.

sjj 919

Inside of the fruit also seems fine