You should declare ALL income both investment and employment income so that
1) only one set of tax free allowances are considered and
2) The full tax position is reviewed, which includes rate bands, incase the combined income sees you into the next tax band of 40% tax (If the combined income is more than £42475.
3) Because you have income which has not had tax deducted from it, that needs to be considered, and self assessment works when a TOTAL review of income is considered.
However, may I ask, will you be bringing the rental income into the UK?
I am not sure if I'll be bringing the rental income into the UK or not. It will be paid into a foreign bank account and I may bring some of it into the UK. If I may ask - why do you ask? Thank you.
I ask because if you are not a British citizen (which I am assuming you are as you state you are returning to the UK after living many years abroad) then you do have the consideration of being taxed under whats known as the remittance basis,but I do not think this will apply to you, so you will need to complete self assessment and declare this income, whether or not it comes into the UK.