Leak at back of toilet pan


My toilet has started to leak on the joint between the outlet from the back of the pan and the large diameter 90 degree elbow that connects to the soil pipe that sticks out of the floor.
I have managed to remove the elbow - it has a multi-fin push on rubber joint where it goes in to the soil pipe (this was not leaking)and a single fin rubber collar at the pan end. I think this collar was originally attached to the inside of the elbow but it appears to have perished and is no longer attached - hence the leak.
I've never dealt with one of these before, but I assume I can just go to a plumbers merchant and get a replacement elbow, including a new seal. If so, is there anything I should know about putting this all back together?
Should I replace the multi-fin seal as well while I'm at it? Does all of this stuff just push together dry or should I lubricate it with vasilene or something?
Any advice or warnings about possible pitfalls welcome.
TIA
Steve
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Steve wrote:

The seals come as part of the replacement. Last time I put one on I just used washing up liquid as a lubricant

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