I'm not sure that there is anything that actually speeds up the rotting process?

If it is not near other plants or things that might burn, you could try partly excavating it and keeping it completely covered to get it as dry as possible. Put kindling around it and burn it, adding wood as necessary until burnt out? Just a thought...

I was told that if you drill holes in the stump and pore in diesel then that will aid the rotting process, be careful though don't spill it on your plants when you pour it. I haven't tried it just heard about this!

Don't use diesel, as you'll be illegally contaminating the environment.
There are products on the market that are for rotting tree stumps.

Or you could drill the stump and pour night water (urine) into the holes.
p.s. don't do this from the source as you might receive complaints from the neighbours, collect it in a container first.
Urine contains uric acid which will kill and rot the stump.