I just bought a little run around Ford Ka Luxury 2004 plate and decided to do the basic servicing myself.

Tonight was spark plugs. I got them out reasonably easy and replaced them with the new ones but when I fired the engine up it didn't sound right at all and was making a low rumbling noise and seemed to lack any power.

I've changed them back to the original ones having given them a bit of a clean and it seems to be back ok again.

I'm pretty sure that the leads were on in the right order but I did notice that the original plugs were Bosch and the new ones I put in NGK.

Could that make a difference? If one of the leads were not quite connected right would it have that effect?

The new plugs being a different make could make a difference. The fact that the problem goes away with the original plugs supports that. Have you checked the gap of the new plugs? They should be gapped correctly but time after time people on here have problems with incorrect gapping

Yes, the HT leads not being connected properly will cause problems

I have had issues with certain engines not liking certain brands of plugs. As above if its running ok with original ones then perhaps its your new plugs.