It is possible that Wesley spoke on this site as the Methodists often built on the site of the previous meeting place.
John Wesley preached in St Agnes, controversially, around fifteen times. For several years there were two excellent Evangelical Anglican preachers in St Agnes and Samuel Walker (see this website) always felt that Wesley should not have preached in towns where the Gospel was already being preached by Anglican clergymen.
In his journal he writes, 'Sunday, July 5th 1747, we rode thence to St Agnes. At two I preached to a large multitude of quiet hearers, many of whom seemed deeply affected. Yet soon after I had done some began to divert themselves with throwing dirt and clods. Mr Shepherd's horse was frightened at this, and, as one of them stopped down, leaped clear of him. The man screamed again; but, finding himself not hurt, he and his comrades poured a shower of stones after him. Knowing nothing of the matter, I rode soon after through the midst of them, and none lifted up a hand or opened his mouth.'
Wesley always told people to go to the Anglican church before they met on Sundays. He wrote in his journal on September 8th 1760, 'The congregation at St Agnes in the evening was, I suppose, double that at Port Isaac. We had near as many on Tuesday the 9th, at five in the morning, as the preaching house could contain. Afterwards I examined the society, and was surprised and grieved to find that out of 98 persons, all but three or four had forsaken the Lord's Table. I told them my thoughts very plain. They seemed convinced, and promised no more to give place to the devil.'