Thanks Rob for your detailed reply.
I already have a couple of F007TH sensors and a Pi Zero ready to ask for thermometer data over 5v I2C, so a simple arduino setup to receive the signal, decode it and await the Pi's request is all I need.
A Uno is rather large for an embedded component so I've gone for a Nano clone. I nearly bought a pro micro as its cheaper and smaller but then realised that it makes it difficult to use I2C (apparently D4 and D5 aren't exposed) and also I would need to buy an additional programmer for it as it doesn't have a USB connection.
I currently have a PC connected by USB to a Nano 3 clone CH340 (with the right driver!) connected by D8 to a 433mhz receiver. I have downloaded BaronVonSchnowzer's Temperature.ino and immediately got it working as it sends output to the USB serial device which I can view using serial monitor in the IDE.
I would like to create an I2C slave device but it may not work very well as the RF receive code is timer based. So is there a good reason for it not being interrupt driven? Is it just that state machines are scary? There is already Arduino code out there to do IR receive which uses interrupts, maybe I could adapt that.
PS I've got surprisingly good range having only attached a 17.3cm length of wire to the receiver board. I was considering making a dipole but I won't need to.