If you want to visit Port Isaac, I suggest you plan a 2-night stay there, as it is delightful, and fun to visit all the Doc Martin sites.
We did that in May of 2014, taking the train to Bodmin Parkway station, with a pre-booked taxi from there (£30). The direct train (no changes) from Paddington Station takes 3h 51min.
We stayed at Orion House B and B right in the village, which was a great choice. The proprietress told us some great stories while she made our breakfast. "Bert Large" was staying across the street right before we arrived. The town has prospered with the Doc Martin tourism, and is very welcoming. Most of the visitors seem to be American or Australian.
At that time, (2014) season 6 had been completed, and there was uncertainty as to whether there would be a Season 7. Apparently Martin Clunes was pushing for it, and they did indeed return in 2015 to film Season 7. Now they are up through Season 9. They do most of the filming in April, so you won't see that in July. But you may see some of the actors in town as they have moved to the area to live.
The location of the village is very scenic, as you know well from the movies. We poked around the narrow lanes of the town, enjoying the gardens, and also walked west on the coast path among the cattle, looping past the upland building (a large barn-like structure) where they film many of the interior scenes. We also walked east to the next village where my husband went for a swim in the bay. If you are strong hikers you can take the bus to Tintagel and walk back from there. We did not do that as the bus does not run on Sunday.
On our return to Paddington Station, when we exited onto London St., we found quite a commotion at the intersection with Praed Street. And then we saw the cause: two contestants from The Amazing Race (which I have never watched, but I knew about it) were frantically trying to find their way through the crowd and into the station. Maybe trying to catch the Heathrow Express to Heathrow for their next stop?