Before I make another purchase, is there any difference between a standard DMX cable and a 5 pin male to female speaker or amp to speaker lead. They look the same to me, but is this correct?

Yes there is a DMX cable is like a micraphone cable and should be sheilded a speaker should be thick i use 2.5 mm flex so there is a big differance

Absolutely no similarity at all. A DMX lead is a twisted pair 120ohm data cable with overall screen. Speaker cable is much heavier and designed for the very high peak currents seen in pro speaker setups (as much as 70 amps for an instant...restrict this and your damping factor etc will suffer).

They're usually both black and round if that's what you meant!!

I have to admit to using 3 pin XLR to XLR mic cables to connect DMX lights in master/slave configuration with no ill effects.

Yeah you can use balanced mic cable for short DMX runs but the post was about speaker cable.

Is there any difference in buying a 6m dmx cable and using it as a speaker to mixer cable ie from Mackie SRM 450's into a mixer?

It depends. If the DMX cable is proper DMX cable and you use the balanced out from the mixer to the balanced in on the mackies then it'll be ok. In this case, the DMX cable isn't actually being used as speaker cable, but balanced line-level signal cable.

Don't ever use it from an amp to a passive speaker...use Speaker Cable.

I used to use Vann Damm XLR cable from mixer to mackie... As far as am aware it was the same cable as I use for DMX-ing.. It all came from the same bloke..

For most mobile DJ setups and moderate sized lighting rigs, good quality mic cable will do the job just fine. Although the DMX standard is 120R, you'll find that even the stuff being sold as DMX cable rarely tops 110R. Decent mic cable is around 90R so not much in it.

I did a main festival stage last year using only mic cable, as we do for all of our events, and have never had any problems apart from the obvious failures with any sort of cable. Be careful of cables made up with screen to chassis though, DMX doesn't like these.