Probably get a replacement chain from Irwin, then join to the existing one... ?
How much does it need extending?, If only by a small amount you could put a piece of bike chain spacer between the grips and the existing chain,
I purchased my Bluepoint Vise grip chain wrench from my local Snap-On man some 32 years ago, and then 3 years or so later I purchased the extension chain from another Snap-On man.
The cheapest option may be to purchase the second hand Vise Grip Chain wrenches currently on offer on eBay. That'll work out about 50% the price of shipping one in.
I'm sure that could easily be adapted to take the Bike/moped etc chain so that it was adjustable so that the pegged operating chain lined up with the jaws at a length to suit different sizes of pipe,
Have u got a link cos I can't seem to find any on flee bay
http://www.davronline.co.uk/product/20EXT-Extension-Chain-18in-for-20r £16.21 +vat = £19.45