Mulsanne Speed idle issue

Just happened to see the British section on here. Earlier this year I bought a 2018 Mulsanne Speed for my birthday. It's a one prior owner car serviced at a Bentley dealer for it's whole life. Bought it with 30k miles and it now has 33k. I've encountered a funny issue with it where it's idling a bit rough. The dealer replaced the spark plugs last time it was in for service, and said these TT 6.75 V8's can foul plugs. I forgot to ask what the plugs looked like after they pulled them out. Dumb me. As soon as the engine is given any throttle input at all, it's butter smooth again. Any thoughts what I should do? It didn't sound like the dealer had any concrete ideas.