UK-Belgium tax issue

You put it on the Belgian tax return, quite simple. IF you are under UK self-assessment, you put in Belgian income onto the UK tax return too. Declaring income does NOT necessarily mean paying extra tax. Use the free tax clinics and the staff there will do TAX on the WEB return for you, annotating it that they've seen proof of your UK income, if it's relevant to calculating tax on Belgian income, it will eventually by converted into euros. You don't really need to pay an accountant, depends on how complicated your incomes are, you should be able to use the free tax clinics.