Close coupled toilet leaking overflow pipe joint?

The overflow pipe is fixed to the bottom of the cistern and water is leaking past the joint.
I have tried to fix it but because of the position of the toilet right in a corner, I can't get a good grip on the plastic collar that pulls the pipe down, making the seal - any advice gratefully accepted. There is a rubber washer on the inside of the cistern and I wrapped plumbers tape around the pipe that protrudes through the cistern before tightening up the collar but I could not get the collar tight enough.