How to untighten a basin waste locknut behind a pedestal basin?

18 Jul 2023
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United Kingdom

I tried to install a new basin waste but I'm not very experienced. I placed it with putty but it hasn't sealed properly and it's leaking. However, trying to remove it and try again with another form of sealant, I found out that i can't untighten it. I tightened it with my hands, and tried to untighten it with a cloth, and then with plastic gloves to no avail. It is a very tight space behind the pedestal and unfortunately I cannot use any of the tools I have. Also, the pipe nut below is very tight too.
Any suggestions?

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Search for basin waste box spanner size is for 1 1/4 nuts
Take out waste clean/dry thoroughly we use white silicone to seal under waste flange .
Should have some sort of washer above nut good bead of sealant on that .
Only needs to be nipped up.
Some will advocate plumbers mate as a sealant I personally think it's the work of the devil.
Waggling the pipe about, might help ease the white nut. A pair of pipe grips might work on the black one.

Once off, try using a thick bead of Plumbers Mait, as the seal, with surfaces dry first. It's none setting, and should squeeze out from the sealing surfaces. You can then cut what squeezes out away.
Some will advocate plumbers mate as a sealant I personally think it's the work of the devil.

It's spelled 'Mait', and it seals where nothing else will, it's just horrible stuff to try to use. Roll it into a bead, to apply it, then struggle to make it stick, but not stick to your hands.. Hours of fun to be had, but what's left, does not go off.
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and thank you both for your reply! I appreciate your help.

It does have a washer above the locknut indeed, this is where I put the putty as it was suggested elsewhere. I'll try to remove it entirely once removed. I'll try to waigle the pipe, too.

I'm planning to get this:

would it help??

Also, I got this in order to avoid silicone and putty

Thanks again for taking the time to answer to my query!
The basin mate seal kit from screwfix should give you a good seal but will need the backnut to be considerably more than hand tight. If you can't get tools onto it, it won't work. Can you move the pedestal? I usually loosen the basin securing screws and get a shoulder under the basin. Hopefully you should be able to lift the basin enough to wiggle the pedestal out allowing better access.

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