You’ll be incredibly lucky to find a valve tester for £100 let aline one that works! I bought the little Orange Amps one (about £350) recently, but I don’t think it does 6SJ7s. Its very limited in the valves it tests, just common guitar amp ones, but it does all the ones I use aside from rectifiers so was worth the investment.
Like to see how you get on with that fella. I bought one a few years ago. It's passed quite a few known dead valves. I'm rather suspicious of it.
I don't have one and don't want one... and I have a mate who has several and would just give me an Avo or Mullard if I asked.... And this from someone who does this for a living.... I don't own a chocolate fireguard either!