[UK-CONTEST] Using DVK with N1MM and FT1000MP

[G3ZQH]

I'm looking for a simple (and preferably cheap!) way to implement digital
voice keying to improve my set up for SSB contests (and to save my voice and
the complaints from family members during 48 hour contests...). Hopefully
there are some experienced hands who can give advice here. 

The commercial interfaces (eg MicroHam) seem very expensive (£250+) so
really looking for a more DIY option. CW contest keying works well and
easily with N1MM/WinKey/FT1000MP but it seems much more complicated to be
able to record audio direct from the rig mike as and when needed during a
contest. I'm thinking along the lines of taking the mike audio from the back
of the rig and using an isolating interface to the laptop mic/phone sockets
(I have an old G3LIV Kenwood interface which might be adaptable- but no info
on wiring etc) but I don't have enough knowledge of how to put this all
together.

So any advice would be very welcome on interface circuitry which is
relatively easy to assemble or perhaps a more affordable commercial
interface which I'm not aware of?

Thanks

Dave, G3ZQH