Yes I thought that the bell-wire filter was on the removable front part.
I was aware that Pressac is the manufacturer (or one them) for BT, so it's a name I respect. I'm inclined to avoid the ones with MOVs (or whatever) in them - am I right? The run-it-direct units are hellishly expensive because of the huge postage costs compared with Amazon, but the second Amazon one I linked to had no details at all.
Would our BT man be ok with my supplying the units?