Printing Problem: Canon i865

I am having a problem with printing from my Canon i865. It worked fine up until Friday when it suddenly stopped printing black. The printer has two full black catridges and strangely, any pages containing colour and black print fine; suggesting that the ink flow is fine :confused:

To rule out a problem with the PCs hardware I tried connecting it to various other PC's and laptops, all with the same result.

I am convinced that this is a problem with the printer. But what? Has anyone seen anything similar or have any clever suggestion?