Is this a manganese deficiency in my lemon tree?

Do you know what rootstock it is on? , usually it says on a blue label on the trunk when you buy them, different rootstocks need different pH in the soil

It is obviously a deficiency of some sort..., does look like manganese could be zinc or possibly iron related as well most common cause is the pH , try adding some sulfur around the tree, alters pH slowly and gives sulfur into the soil that is also needed as a micronutrient
A lot of Eureka trees look like that due to the rootstock they are often on, every one I have ever bought has been delivered with pale mottled leaves, with heavy fertilisation (including micro nutrients ) they always recover

but could be pH related as pH incorrect ( too high or too low ) stops them taking up micronutrients , if you spray citrus specific fertiliser on that says it can be sprayed on it will absorb rapidly through the leaf and correct deficiencies , the one from `The Citrus Center` seems best , just spray every day until it recovers.

They do need a lot of fertiliser, even in the ground , to add constant micronutrients add some extra fertiliser from a slow release that has high levels of micronutrients required, add it as extra and still use the full amount of citrus fertiliser as well.

How are you going to keep it warm in the winter? it will not grow well outdoors in the UK?

Thanks Starloc - great reply as always.

1) No idea on rootstock - it was bought many years ago in a sale, long before I realised the importance of noting such features!

2) I have probably underestimated the need for constant feeding when planted in the ground, and shall feed with a citrus foliage spray every other day for the time being. I'll fill up with citrus centre feed once I be exhausted my current vitax one.

3) As for the winter. I am lucky enough to live on the south coast, so cold weather is rare. My other citrus in containers live outside, but under a frost free canopy, as they seem to do much better than coming in, where the caterpillars have a field day. This one in the ground is sheltered, against a warm, south facing wall, so will be enclosed with a temporary greenhouse (to keep frost free and dry) with extra heat added if any real cold weather is forecast. I have a passion for tender exotics so am getting used to winter protection!

A lot of Southern England sits on chalk, so maybe your soil is much more alkaline than you thought, and the excess calcium is affecting the uptake other other metallic nutrients.

I don't grow any citrus species, but the leaf symptoms look like Iron deficiency to me. What iron there is is translocated up from the roots into the leaves but fails to move out of the veins - hence the dark green colouration (indicating sufficient iron in the tissue) is limited to the midrib and main veins.

I agree with starloc regarding the use of a trace metal spray. This would cover manganese as well. Alternatively, Iron could applied in a powdered chelated form (e.g. Fe EDTA) to the soil.

it's a zinc deficency:

Leaf Diagnosis

I'd be inclined to buy a liquid fertilizer such as Advanced Nutrients Sensi Cal Bloom, which has a balanced wide range of minerals including Zinc and use it as a soil drench.

Zink deficiency is most often yellow upto the edge of the leaf, manganese most often has a band of green around the edge, Very hard to tell between them, so could be either

Most likely is incorrect pH or lime intollerance causing lack of nutrient availability to the roots measure the pH of the soil

It is probably growing of Citrange Carrizo rootstock ( often listed as incompatible with Eureka lemon , but widly used ) or citrus Volkameriana rootstock, it will not be on Poncirius Trifoliata rootstock as it is incompatible most books list the pH and conditions required for Poncirus Trifoliata and that would be incorrect conditions for the others