If the price of them has come down a shade, and you will know fairly quickly whether it is a sensor problem (IE new print head does not cure situation) my only concern would be whether buying a second one, it may be a design fault and in another 18 months, you could get similar problem on the replacement.
It might be worth seeing if there's a Canon specialist in your area (sorry, maybe the shop I was thinking of only deals with photo-copiers!) ... OK scrub idea of shop in town...
Is it worth talking to Canon about using a courier to send your printer back for them to check, and get a further warranty?
If that is the case, you could
- buy printhead, see if it cures fault.
If not then
2) buy a new printer. Same model, keep printhead along with ink cartridges. Be 100% sure the replacement printer will be compatible. Would be a pain if they had a Revision B or Revision C model of the ix4000 and had internal changes and some new cartridges etc
whether the new printer takes the ink and print head or not, return ix4000 to Canon for them to check over, on condition they will give you 3 or 6+ months warranty. The cost should be lower (even if sending it each way costs you a total of 30 quid) as they can surely not charge much more than 20 quid an hour (while call-out of 105 covers time driving to/from, fuel, maintenance, insurance, and sending it to them means it just goes into a workshop with lots of other printers).
- sell refurbished/ checked printer and use warranty against problems as way to ensure it can be sold as working (eg on Ebay). Make copies of any documents from Canon and after buyer has had printer 3-4 weeks, forward original document on. Buyer has had time to check goods, and be using printer so will have no reason to complain about delayed paperwork even if they think it hints of having had some problem(s).