Electronic – RF Receiver for USB

It's funny how writing a question helps one think through a problem more thoroughly. It turns out there were two issues:

1. Although I'd seen this link before, I realised I had still made the same mistake; I'd tried a much lower baud, rather than much higher baud, than the bridges support, so it would have disregarded my command like in the link. However, changing the baud rates through device manager to sensible (but different) values, they still communicated as if on the same baud rate.

2. What finally got it to work, was not setting the baud through device manager, but through hyperterminal (or putty as I'm using). It's always the obvious things!

Something along the lines of a couple of Alpha-TRX433S would probably do.

They are simple transceivers with an SPI interface. If you want cheaper eBay is your friend - this RF-2400P looks to do the same thing as the above, but won't come with a nicely written datasheet.

EDIT - It turns out this one does have one - User834 has kindly provided a link to the R4-2400P Datasheet.
Anyway, the reason for the initial assumption was that often the minor electronic products sold on eBay don't come with anything much in the way of documentation, which is something to weigh up when choosing where to buy from (plus waiting times, customer support, etc)
EDIT 2 - Along with the datasheet, on their site they have some other RF-2400P documents/code examples (see pages 2 and 3) which may be of interest.

You haven't mentioned exactly how the bot is controlled at the moment though. I am assumimg you can interface with the ATtiny's SPI.
If not let us know what you have available on the bot to interface with.