Loft hatch is in bathroom
Tannith - Do you have an appointment for the plumber? In your shoes, with your concerns, this is what I would do.
Firstly, on the appointed day, clear as much as you can from the bathroom - toiletries etc (to make it super easy to clean afterwards). Then, I would ensure I had a mask and gloves for the plumber. I don't know how your house is configured, relative to the public footpath, but I would then put the mask and gloves in an envelope, with a note to the plumber.
In the note, I would outline the arrangements. He rings the doorbell, you open the latch and go into a room where you will stay. He lets himself in.
Your note tells him where to go in the house, and he should telephone you to discuss what the problem is or anything else he needs to discuss.
He gets on with the job.
In the bathroom, leave out anti-bac wipes or whatever, for him to wipe over the hatch when he finishes. Have the window open for as much fresh air as you can.
He calls you when he has finished. He lets himself out, and goes onto his next job.
When he goes,don your best marigolds and give the bathroom a good going over. If you have cleared the surfaces etc., it should be very quick and easy. Leave the window fully open
Pay him over the phone.
That's straight from the hip, but it's certainly doable. You can be in the room next to where he is working, if that makes you feel safer, or go as far away as you can in the house, or garden.