So I thought I would finish this off - for any future readers (or if anyone is still interested!!!).
I did buy the 1000w dimmer - but as I was advised that dimmers see the full load of "conventional" electronic transformers, then my 17 x 60w transformers would have exceed 1000w to the tune of 20w.
So, along with the dimmer I bought 2 varilight transformers, which have been designed to only load dimmers with the bulb wattage they consume - and not with the maximum load of the transformer. So with my 20w bulbs, that gave me a total load of 940w on the dimmer - safely inside the 1000w limit (when you are counting loads at the transformer you don't need to worry about derating - in fact, I understand that's what derating tends to account for; the discrepancy between transformer load an bulb load).
So with the new transformers installed, I plugged in the new dimmer, and hey presto - it all works!!! No 'capacitance' issues, or harmonics. The dimmer buzzes very quietly, but I can happily live with that!!!
Thanks for everyone's help with this...