Many many thanks, Simon, to Ilford for still manufacturing multigrade filters in various sizes. I use the filters in condenser drawers well above the negative and lens stages, and have found their various dye stabilities change remarkably slowly with age and use. I always buy a pair of filter sets at one go, date the different filter boxes, and use one set until I can visually notice filter-fading. Then I replace with the other 'new' paired set, toss the faded filters, and buy two new Ilford sets.
Thank you to all...this has been very helpful.