I've had a reply from the RHS about the Caliente Mustard....

Thank you for your enquiry to the Royal Horticultural Society's Members’ Advisory Service.
Please find attached our fact sheet on green manures, herb suppliers and vegetable seed suppliers.
I have unfortunately been unable to find a retail supplier that supplies seed of 'Caliente' mustard, although Edwin Tucker and Chase Farm Organics both say they are considering stocking it next year.
You may find that the usual species of mustard sold as green manure (and available form the suppliers in the attached factsheets) confer many useful benefits. The additional benefits of the'Caliente' variety, which is cut and covered with polythene in the sun to allow biofumigant gasses to be released and help to sterilise the soil, may not in practice be effective in the average home garden situation. I have discussed this with our plant science department, and they say that a very large surface area needs to be covered in this way for the process to be effective at all, and that it is really designed for large agricultural fields, not domestic flower or vegetable gardens. It is not possible to build up enough heat or a high enough concentration of gas when there are as many 'edges' to the area covered as there would be when only a relatively small area is treated in this way.
However, I will follow up this matter with the Trials Department and we will monitor the project for applicability to the home garden situation.
We have forwarded your comments and suggestions on the absence of this seed from our own RHS shops to the Wisley Plant Centre, and you may receive an additional reply from them.
I hope this information is helpful, and thank you for bringing this matter to our attention, as I am sure we will receive further enquiries on this subject now that Caliente mustard is being showcased in our own trials gardens.

NB I find it a bit odd that they are trialling something that is recommended only for agriculture when their main appeal surely is to home gardeners....but I'll be interested in whatever they come up with.
If anyone is interested in the list of suppliers and green manures I'll copy that to a message.