Leaking BSP joint on shower waste - what to try next?

[jack]

We've had a leak underneath an upstairs shower for a while now. I've finally gotten around to opening up the ceiling underneath and having a look.

 

There's a 135 deg, 1 1/2" BSP to 40 mm compression elbow coming straight off the 1 1/2" BSP outlet from the shower waste. Based on the muck around it and how clean everything else was, it's clear that's the source of the leak.

 

The BSP nut was very loose - I had to turn it nearly two turns to tighten it up.

 

I hand tightened it up as firmly as I could and tested: immediate leak.

 

Dismantled and pulled out hair, gunk, etc from the shower waste outlet, then cleaned up the fitting and connections as best I could.

 

Installed again, firmly hand-tightened everything, tested: immediate leak at BSP joint.

 

What would be your next step? My first thought is that the rubber washer's hardened over time, so replace that and see what happens.

Anything else to look out for? Should I go for the same square section washer?

@Nickfromwales, tagging you in for visibility.

 

Thanks.