Hi Anthony Chambers,
Where you are not UK resident and have income from UK property, you meet the criteria for completing a self assessment tax return each year.
In the tax return, you declare your UK income.
That might be things like UK dividens, bank / building society interest as well as the rental income and expenses.
The NRLS scheme, where approved, allows you to have your rental income paid gross.
Where not approved, basic rate tax should be deducted from the monthly rent and sent to HMRC.
Usually an agent or the tennant would do this.
In either case, a Self Assessment tax return is required.
As you don't say how many years you may have been renting out the property, you may find looking more information at the let property campaign help guide at:
Let Property Campaign
Thank you.