@Euan Welcome to the community forum and it is lovely to have you here 
Thank you for taking the time to share your feedback about the map and search locations.
I had a quick look to assist you further and it might help to set up saved searches in Yorkshire, North Yorkshire and East Riding of Yorkshire to make sure you cover all areas that you are wanting to sit in.
Here is what the search terms look like in the search bar:
I did a lovely housesit with Oreo the Cat in North Yorkshire previously, but am not from the area so did a quick Google for Hull and that came up under East Riding of Yorkshire.
I am originally from Sussex and did a quick search for Sussex and the search results are split into East Sussex and West Sussex. So it looks as though the map is picking up the locations as individual counties.
I am always keen to learn about the country I grew up in and I always though that ‘Yorkshire’ covered the whole are, had a quick Google and was surprised to learn that it was split into 4 counties!
So it might be that the map is functioning that way? I am more than happy to pass this feedback on for you and if you would like to add the link to your THS sitters profile to your forum profile so that others can view it and connect here is how:
https://forum.trustedhousesitters.com/t/how-to-add-a-listing-or-profile-link-to-your-forum-profile
I can also do that for you, just let me know 
There is also a couple of great threads:
You can add any suggestions to these and they will be viewed by the product team.
I hope you manage to find a great sit in Yorkshire and keep us posted as to how it goes.