After you dig the hole, any impermeable barrier you put between
the compost in the hole and the surrounding soil will prevent
leaching across the "border". Also you may want to create a
barrier of some sort to keep the roots within the ericaeous compost.
This barrier will be buried anyway - just sticking out at the
top so can be cut out of old compost bags, old growbags, any heavy
duty plastc sheeting you can find, old polypropolene tarpaulin
whatever. So just line the hole with any of the above, if you
dont have anything else to hand, and they should do just as well.
You just need an impermeable barrier of some kind. Whether heavy
duty plastic will contain the roots is another matter
as I'm not familiar with the rooting habits of blueberries. And as
its buried and away from any UV light plastic will last for ever
in that situation, and nobody but you need ever know its there.
Then you can adjust your feeding and compost mix in the light of
the results you obtain each year.

Good idea - I'm looking forward to learning all of these tricks ! I have
a fair number of heavy duty bags around - left over from the horse
manure that I obtained from a local farm. I'll give them a go. Maybe
even use a double barrier incase the routes are particularly aggressive.
Thanks again!

ps - just tested the soil with a kit from B&Q, looks like its more on
the side of alkaline than acid, so even more of a reason to use proper
compost and a barrier