Get a man to do it. Washing up sponge (scouring side), brute force and rinse under the tap.
Washing soda is sodium hydroxide and making a solution at home would tend to produce something more concentrated than the stuff you may remember from science class at school which made your fingertips feel 'soapy'.
It's great for turning grease into soap but I'm not sure it's having any effect on tea-stains, Maydup, so you could save money there. Then again, any rats coming near your house, up the sewers, will get a nasty surprise! ;)