Thanks all for your input and advice, especially jayc you have been most helpful with this matter.
Today I plonked a 9 litre bag of cal aqua labs black earth premium substrate (normal size) into one half/side of the aquarium (distracted the shrimp across to one side with treats first of course and shooed away one or two stubborn ones)
The packet mentioned nothing about rinsing, in fact the instructions said little more than words to the effect of "put substrate in tank and add rocks and plants"
In all honesty it did make the water cloudy but it was not significant and it cleared up within half an hour I'd say - the shrimp didn't seem bothered by it in any way. There was a negligible amount of miscellaneous organic material in the bag
Needless to say I am very pleased with the minimal disruption to the tank and if all is well over coming days andweeks then I would highly ecommend the product to others..
My pH hadn't risen any higher 6.8 from several days ago, I'll watch how it progresses over the following days. I can't imagine there would be any dramatic change with just one bag but hopefully it will prevent any risk of further pH increases. If ammonia and nitrite are still at zero after a fortnight i will hopefully be able to afford to grab another bag to cover the other half of the tank, plus I will still need one more later to have the recommended amount for my tank size.
I must share some happy news - while pouring in the substrate I disturbed a moult and i thought could there be love in the air?
Lo and behold this evening I discovered I am nowthe proud keeper of a berried bee shrimp! I'm very pleased, it's only been 8 days since receiving them from heavyD and already there is joy. :-)
love n peace
will