In my opinion Vagabond, the PH is not the issue here (since you're using soil)..unless the PH of your tap water is particularly bad, but really you scots seem to have some bloody good water up there! lol
The soil will stabilise itself at normal PH levels..it's when the water used is way out that any adjusting should be taking place really.
However some growers do, and if you'd like to then I know biobizz now does an organic PH down which would be suitable for soil.
The issue I see, is that you've chosen the wrong john John Innes..you would have been better off going for John Innes #1 or #2..the seedlings might find the #3 a little hot! (in the way of too much nutrients).
I used to use John Innes #2 with decent results back in the day
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