Hi Peter,

I think you have done very well if you still have the original mylar seals.
I have also replaced the seals with tape, and yes, it is better. It is also quite a bit of work though.
The tape goes on the aileron and then in an S to the wing. It needs to be glued in place, and is easiest if you can remove the aileron to do this. It simply gives you easier access to it.
You can happily put mylar seals on over it, and depending on how your Discus was done in the past will determine what needs to be done. Some have a groove milled in only the top surface, and some also on the bottom. I personally reccommend you put mylar on both surfaces.

Next tip is to put deviders between aileron and wing to stop any airflow between them. Adds to the effectiveness of the aileron too. I did it both inboard and outboard. A piece of 1.2mm carbon worked well for me.

Don't know how to test the rubber seals.

Hope this was helpful.
Cheers
Ross