Posting this late as i wanted to know the Spawn bag was going to work before I added on here.
The Rye Grain spawn bag worked a treat, initially it took a few weeks to see proper signs of life then once about 10% of the bag was colonised I mixed it up thoroughly, colonisation was then complete within 2 weeks.
Yesterday I prepared the bulk substrate; 500g Coco Coir, 500g Vermiculite & 100g Gypsum (Calcium Sulphate) with 4 Litres of Boiling water, left overnight to cool, this recipe was a bit too wet so I assume the dampness of the Coir I used was the reason, I squeezed out each handfull of the mix before adding to the Monotub.
Today I broke up the spawn bag and mixed thoroughly with the bulk substrate in a homemade Monotub.
Fingers crossed for a good further full colonisation.