Replacement for bath trap leaking onto ceiling

[deeceeb43]

Hope you can help me with a novice query - the current trap sits in a cut-out in the floor under the bath. No matter what I do it's weeping from the thread on the connection to the waste.

I think it was cross-threaded at some time as it has been stuck on to the old waste with mastic so I want to replace it just in case there is ever a flood.

Waste pipe connects directly to the stack. The trap connects to the waste pipe by screwing onto a small piece which then connects to a push fit bend.

The local plumbers merchant told me the old trap was a P trap which was the wrong sort for a bath and sold me a McAlpine shallow trap which I now know has a 19mm seal that's too low. I see posts about 50mm or 75mm being the minimum and I'm not sure which I need.

Which of these is the correct way to go please?

http://www.mcalpineplumbing.com/traps/bath-traps/1-189-x-50mm-seal-bath-trap.html

http://www.mcalpineplumbing.com/traps/bath-traps/1-189-x-75mm-seal-bath-trap.html

Thanks