Got a couple of answers from my consultant today! He said if I have to miss one dose of rivaroxaban due to a tummy upset it doesn't matter; more than one probably does and if I have an extended period when I can't eat, to get in touch as I'd have to have heparin injections. Just what one wants with an upset tummy, not!!!
I had food poisoning (mildly) just before Christmas and it dragged down my immune system, so I've been unwell on and off (only once badly enough not to have my rivaroxaban) most of January. Think I'm finally recovering now, though, but I won't be going to Buffalo Grill in Calais any time soon, or if I do, I won't be eating their "Welcome salad" (it had, I think, sat out under hot lights slightly too long.....).
Dr M also confirmed that one should not eat grapefruit while on it - he had to look it up, as it's not on the UK leaflets, but apparently is in some other countries, so best to avoid it.