it's a TW wagon, 99k, dual fuel, regularly serviced. All plugs are iridium and not very old. Air filter element about a yr old. Over the last few weeks it's got v hard to start, cranks strongly for ages then reluctantly starts. I used a fully-charged jump-start battery as well to see if this helped, no difference.

Any suggestions re what the possibilities could be?

Thanks

Does it start better on fuel than on gas?

it's the same on petrol as gas for starting. I'm thinking high tension leads so I'll try a new set.

Issues with leads are pretty rare round here...

fitted new leads; it is much better, now seems 100%, hopefully problem is gone for good.