Thanks for your question.
You should be declaring this rental income through your UK tax, by completing a foreign income page along with self assessment.
Along with that an employment page to ensure that all income is declared.
From the rents received you can claim expenses such as interest on the mortgage used to purchase this property, maintenance fees, insurance, advertising, cleaning, accountancy fees, managing agent fees etc
As you have been receiving rental income since 2005, it could be prudent to engage an accountant/tax adviser to prepare the required tax returns for you. Be aware there could be late charges added onto any tax due - and also interest.
The losses you have made against lost value on equities can only be offset against capital gains - so this could not come to bear until the French property is sold - or you sell something else that gives rise to a capital gain. (These cannot be offset against rental income or PAYE income)
However do declare the capital losses as you can carry these forward.