Hi Janj

[..]

However, within a few weeks the water started to smell. I also noticed
a number of small grubs in the water - some of which have grown to the
size of maggots. The water also has the oily surface that you get on
stagnant water.

Did the maggot look like this?
http://www.cals.ncsu.edu/course/ent5...ges/20_jpg.htm
If so, harmless, but still not pleasant to run into.

Yep - that's the critter.

[...]

IMO, the grubs didn't come with the soil, a mature fly laid eggs and thus
they hatched. You can either add something that will eat them, mosquito
fish, tadpoles, or small goldfish or you can use a little bleach (2ppm
won't hurt plants and it will be gone in 24 hours, if not sooner as it
reacts to the organics in the tub).

Thanks for the tip - a few drops of bleach have sent them all floating
to the surface.

Regards,
Colin E.