Success! Again, thanks for everyone tips and advice. It worked.
I did the coffee slurry bit. Then drizzled a fair bit of Bailey's directly on the bottom half of the cake. Middle layer was a double cream with Baileys & icing sugar and the top was butter cream frosting with Bailey's. Topped with a Bailey's chocolate for each slice.
It went down a treat. Everyone LOVED it. Even my 6 yo boy who said he wanted this for his birthday cake. I won't say it was light but it was not as heavy as I thought it would have been and the Bailey's taste was just right for a Bailey's cake.
If anyone tries something like this, the recipes from the links provided used 3 tbsp of Bailey's in the cream filling and used the cream in turn to flavour the frosting. Don't bother. This, IMO, would yield a very weak and unsatisfactory taste.
Go for at least 6 tbsp in the cream and 8-10 tbsp in the frosting (basically don't use any milk just Bailey's

). It gave a really strong taste but was not biting. As everyone said the frosting was the best they'd ever had.