Bath tap leak advice

[Moo123]

Recently discovered a long term leak that got through the floor boards downstairs over time. It seems to be coming from the hot water tap, and only occurs after it's been used (rather than a slow leak, but hard to say) and runs down the copper piping on to the floorboards.

Photo ref
https://s28.postimg.org/klwgiief1/IMG_20170122_100039.jpg

The water is coming from inside where the black (backnut?) nut at the top fixes the tap to the bath. I also think that sometimes the hot water comes through where the stem of the tap is when first turned on, but can't be sure.

The plan is to get the taps changed by a plumber, as it looks like we need an isolation valve installed to future proof any changes (currently all copper piping) but given how rusty things are, wondering if it's actually something easier like a cartridge change?

The taps are Bristan Birmingham 1901.

Any advice would be appreciated. Thanks!

 

[dilalio]

It's more likely that the tap internals are fine and water is getting down around the outside of the tap and through the hole in the bath. Have you tried running it and looking under the mounting with a torch?
Try backing the backnut off and applying some plumbers mait around the thread where it comes through hole and then doing nut back up to it. Some clear silicone around the base of the taps would also be good.

Edit: looking at the photos, you'll need to get a wire brush on that lot and clean it up best you can.

 

[oldbuffer]

If that's a pressed steel bath you may need to replace it. It looks as if its been leaking for ages, and may have rusted through around the tap connection shown.