dreadful customer care

I bought a Lenovo all in one PC for my business, we run a Beauty salon and the computer and salon software is critical to our business. When the existing computer started playing up I needed one urgently and went off to PC World (as they are supposed to be the experts). Could only buy a PC with Windows 10 and was told by the salesperson that it is compatible with all third party software, but if for any reason there was a concern I could return it for a full refund or exchange.
Fell for the up-sell when he told me he would do a bundle of Microsoft office, cloud and he would throw in an extra free year of antivirus, all for 80 quid, I thought good deal, he failed to point out that the Microsoft office was only for a one year licence (very clever wording, obviously been on the up-sell training course). Went to purchase and was bombarded with up selling, I agreed to pay for set up (big mistake) for about £45, whereas they opened the box to do the set up.
Installed the computer at my shop and was told by my software provider that it was not compatible, so I took it back to PC World for an exchange or refund (as promised) but now the fun begins. The Manager told me I had forfeited my right to return as the box had been opened (by PC World staff) who did not tell me at the time that this would invalidate my right to return. Now stuck with an £800 computer.
Tried to install it at home and launch Microsoft word but was prompted to enter a product key (odd as I had paid for a set up), tried all product keys and codes no joy, rang help desk (joke) who told me it should work, and questioned whether I had bought office (straight away on the defence, adopt the blame the customer technique, treat them a liar or thick) and was told to go back to PC World for them to sort not pointing out that I only had 30 days to do so.
Just finished working away so I decided on one of my rare free Sundays to take the computer back (about ten weeks after buying) to be told their is a software problem, once again PC World straight away adopting the blame the customer technique as in their words they said I must have done something to cause the problem. Best yet they said they would (look at it) yes only look, and charge me£50 for the privilege of doing so. Explaining I had paid for basically a plug and play system expecting things to work with no change in their attitude (basically pay us again or go on your way) I would question if anything was done during the set up as the office program never worked at all (dodgy dealings), they just opened the box to invalidate my rights.
I raised the issue through email to the customer care (don't give a care) line and left my mobile number, few days later received a call from one of their highly trained and experienced customer care staff who adopted the same blame the customer technique and said I must have caused the problem, would not accept that the PC should have been set up to work from day one and I had to pay the £50 because I had gone over the 30 day limit and now they had no responsibility whatsoever to the customer.
I am in business, if I treated my customers with the same attitude I would be out of business very quickly. PC World though seem to adopt two policies:
1 get as much money out of the customer as possible
2 sod the customer as soon as they have left the store, unless they can make more money out of you.
Strongly recommend avoiding PC World, look at other reviews if you want convincing, and let the sods go to the wall.