I bought this AN2131 board on eBay for £12.50 + P&P. It's just a AN2131 chip with the required support circuitry - crystal oscillator, power regulators etc.

First I took the inputs and outputs of the key matrix and wired them to the GPIO ports of the AN2131. This was just a matter of plugging the whole lot into some protoboard. This didn't work too well, as the keys would get "stuck" often. This is because an unconnected chip leg can "float" between on and off due to the residual charge held in the wire. The solution is to add pull-down resistors which allow this residual charge to flow to ground when the key is released. I added one to each input of the circuit. I used a variety of spare resistors from the junk box 320 ohms to 1000 ohms. This is the final circuit diagram:

And this is what the final circuit looks like:

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