As per my comment, I found some affordable AIS receivers here.
The cheapest seems to be this one.
Receiving on dual channels (161.975 MHz and 162.025 MHz) alternately
Auto-hopping channel algorithm improves captured message rate by 4%
Manually adjustable hopping interval rates (0.25 sec, 1 sec, 30 sec and auto-hopping)
Sensitivity down to -104 dBm@30% PER
Up to 12 nautical miles receiving range
USB 2.0 powered (<26mA @5.0V)
Serial output: 38400 bps baud rate over USB
Message output in NMEA 0183 format
QK-A021 AIS Receiver USB Dongle(Auto-hopping V2.0)
SMA connector for 50 Ohm VHF antenna
SMA to BNC coaxial adaptor included
Compatible with Windows XP,7,8,10, Mac OS X, Linux systems
1 x QK-A021 v.2.0
1 x SMA-BNC coaxial adapter
1 x USB extension cable
1 x CD containing Manuals, Configuration Software and driver
For $60 / Eur 55 / £50, I doubt that I will find cheaper.
The only question is what kind of an antenna I would need. They sell one for a further £30, but I will shop around for a cheaper.
For programming, I would imagine that it presents itself as a serial port.
[Update] I bought this antenna for £20, and, yes, the device presents itself as a virtual serial port.
[Further update] it turns out that this particular solution is of no use to me. Despite my living less than 2 miles from a major working river, the device cannot detect a single vessel.
I imagine that I will have to purchase a more expensive device & will post details here. Meanwhile, this is probably adequate for use on open water. It does seem to get reasonabale reviews.