Hello Gail, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your problem.
You are in a spot of bother and no mistake as indeed you should have been declaring the net income. The following expenses [source: Which] band be deducted:
'The most common types of expenses you can deduct are:
The expense should be incurred wholly and exclusively as a result of renting out your property.'
What you must do now is work out for each tax year [6 April - 5 April] your net rentals. Then make your peace with HMRC by writing to your tax office giving details of this undeclared income. Now HMRC can do two things. They will charge interest on underpaid tax which is only fair as they give interest on overpaid tax. They may also charge you penalties for failure to declare. However, depending on your other income sources, if there has been no loss or minimal loss of Revenue, they may be lenient and waive penalties on voluntary declaration. Don't bet on it though!
I do hope that I have shown you a way forward in this matter.