"Charlie Pridham" wrote
All good points, my problem is that I do not know what they are growing
in (and they are too large to consider repotting) they are not looking
that brilliant because they have been out doors with no care or attention
for at least 10 years and were found under a large montana clematis when
it was cut down, so I am keen to get them back into leaf all over and to
make the leaves I have got green all over instead of green with yellow
patches! Once I have got them growing I can prune them back to some sort
of sensible shape.

A dose of Sequestered Iron will help green up those leaves.
Can you check the soil in the pots for pH? They prefer acid soil although
it's not strictly necessary, and won't be if you water with soft water,
however if you use hard tapwater it would be best to use ericaceous compost
when repotting .
I understand the old Head Gardeners always repotted a sick looking citrus
into ericaceous compost.

A neighbour left his quite large Lemon tree outside all last winter, by
spring it looked virtually dead with most leaves dead or gone. When spring
came, after a light prune and a feed, it quickly came back into full leaf
and flowered. Tough plants.

--
Regards
Bob Hobden