I'm crap caught on the hoof too so to speak If you aren't happy with the sitter - ring up and get your key back.
I'm lucky in that I work 10 minutes from home, so I go home at lunchtime, but occasionally I don't and the dogs are fine. Sometimes my brother will let them out for me - but Rudi doesn't even come downstairs those days - he stays curled up on my bed!
I would not have someone walk my dogs but it would be good to have someone just go round to let them out if I couldn't make it back one lunch time. How about asking if, for £10/£15 she'd simply go to your house, let the dogs in the garden for a pee, re-fill their Kongs or whatever and settle them down again.
I really do think that your dogs will be fine left on their own for three days a week, they still have you there four days a week. Why not try it for two weeks - if you see changes in their behaviour i.e. destructiveness, stress etc., then re-think having a walker. I think you're worrying yourself far more finding a sitter than your dogs will stress about being on their own. Give them a good long walk in the morning, fill up Kongs with nice stuff and leave them to settle