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Thankfully we got a delivery on Sunday for all 3 houses, and got another one booked for next Saturday, and local shop also delivers (we really do live in the 1950's), and any other deliveries are left in the garage for a couple of days and then unwrapped out there whilst wearing gloves, so we are doing everything we can to reduce risk, will see how the week goes I think.
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Also depends on where/when each of you mix outside of the properties.
For example if you, in a couple of weeks, went to Shop/Supermarket A and bought some things.
Then, a bit later, if one of your in-laws went to Shop B.
A bit later someone from the third property went to Shop C.
Without the mixing there would be no connection (via your family at least) from Shop A to shop B or shop C but by mixing families you now increase the exposure/mixing risk for everyone else. People who also shopped in Shop C who never went to Shop A now have a possible connection (however small).
By keeping separate you help minimise this risk.
if you do the full two weeks without symptoms and can get food/supplies delivered without mixing with other folk then I don't see why not. Just need to be careful about handling other deliveries.