Help - Cannot remove drain cover

[sooty61]

I have been having slow running drains for a while now and now when the bath is emptied or SWMBO has an extraordinarily long shower in the upstairs bathroom the kitchen sink can overflow. I moved into the house a couple of years ago and it has had a couple of extensions so I do not know where the internal plumbing runs. I have emptied OneShot down the plugholes numerous times and it seems to help but then backs up again. I lifted the manhole on the drive and all was fine, but this one is under the floor in the utility room and I want to check it is ok, but I can't shift two screws. I have tried a big screwdriver, heating them with a blowtorch, and just failed with an impact driver.

Any ideas before I have to roll the dice on potential cowboy plumbers?

[sooty61]

Thanks for the replies. I forgot to mention I also tried soaking them overnight in WD40 before attempting to unscrew. I guess drilling them out is easier than chipping out the whole frame and replacing?

[sooty61]

Ordered some Plusgas so will see how it goes. thanks