Andrew,
You can also buy such packaged RF modems from the likes of Black Box, Brainboxes,
CoolComponents etc. in UK.
Can I suggest one thing you might need to check though before investing too heavily in any such kit, and that is the levels that the RS232 is driven at... "Typical" RS232 at EIA levels has a swing of +/- 12v (from memory, spec allows swing up to +/- 15v and down as low as +/- 3v) but for noise immunity in industrial environments, industrial kit tends to have the detection thresholds set towards the upper end of the spec. However, some modern adapter gadgets (and some USB-serial adapters) can operate with the correct polarity but only swing, say +/- 5v and/or have very modest current source/sink capabilities - and hence whilst the adapters will happily connect two PCs they won't always provide a 'loud enough' signal for an industrial CNC or PLC to recognise.
I had a hardwired link from my drip-feed PC to a TNC155 which included an opto-isolated safety barrier - the hardwired serial had enough 'oomph' to drive it but a BT one didn't. On the bench, I found that the BT units were only driving +/- 4.7v off load and +/- 4.3v into the opto whereas that hardwired ones could strill drive +/- 11.5 into the opto. In that case I was able to bypass the optical barrier as the wireless link removed any possible fault path or ground loop issues.
Dave