Citrus need 20:20:20 in winter and a higher-nitrogen feed in summer.
They don't have root hairs so they need careful and regular feeding.
Don't use tomato feed - a balance too much towards potash is counter productive with citrus (weird, I know).
Chempak used to make two perfect mixes that only needed a bit of extra foliar feed to boost the trace elements to perfection - not sure they still do.
It used to be possible to get an ericaceous fertiliser with extra nitrogen but that's probably gone too.
Basically use a balanced fertiliser feed and alternate with an ammonium sulphate or dried blood feed in summer. Trace elements and chelated iron are fairly easy to source.
Citrus do best on a capillary mat watering system - easy to set up with trays, slates, whatever. If you do this use a plastic pot to make a good connection. If you don't do this use a clay pot to help reduce over-watering (still risky).
Cheers.
