Wow, it's almost a month later. I suddenly remembered that I came here when I was feeling desperate, and had some support, and so I want to report on the follow-up.
Just before I went on holidays the Macbook Pro started to fail again, the very evening before I left. I had planned to make music while on my travels, I have people waiting for tracks and that was very stressful. During my trip the Macbook got worse, so much so that I could not make any music. Logic Pro was especially good at making the graphics chip crash and bad luck, it's what I use for music these days.
Meanwhile, before leaving, I had been confronting the store with my evidence that the board had not been changed. They first said that something went wrong and they would like a chance to get it right blah blah blah, delaying a refund while never plainly stating that the board had not been replaced. After I became somewhat threatening I got most of the money refunded, but not all of it as they hoped to take another look after my return. But when I said I did not trust them enough to give them the computer again, they finally agreed to refund the rest. All good, except since then they still haven't done so. Well, there is a deadline tomorrow after which I'll take it to the small claims court and trading standards.
The good news is, since my return, I have finally been able to get an appointment at a nearby Apple genius bar.
Much to my surprise, and unlike some horror stories I have seen on the internet, Apple charged me LESS than the independent repairer. Not only that, but I left them the Macbook at around 6 PM and at around 10 PM the same day I had a message saying it was ready for pickup!
I think they used a refurbished replacement board, because there is also dust in the ports. But I'm not too bothered, refurbished by Apple is perfectly acceptable to me (and that possibility is in their terms and conditions anyway, my biggest concern was that they might have refused to exchange the board as it had been tampered with some month earlier, by the same independent company, for a graphics chip replacement... which I now wonder if it happened at all).
The super friendly and quick service at the Apple store has restored some of my "faith" in Apple (well, faith is a bit of a strong word, but I'd like to stick with Mac OS for a while, as I have now invested lots of money in Apple hardware).
I know that there is a class action lawsuit in the US about Macbooks, their failing logic boards, their overheating. But I am happy I have mine back, and just hope I get lucky.
My plan is to use it ONLY on holidays and on very rare occasions at home. I have purchased a second-hand Mac Pro for every day tasks at home (including music production), and upped it to 16 GB RAM and a 500 GB SSD. It's an early 2008, 8 core model. Not the latest hardware, but with the SSD and 16 GB RAM it's superfast. And it has a 5570 HD graphics card so I enjoy two 24.1" screens. I love it.
My thinking, and my hope, is that if I use the Macbook Pro for the equivalent of one month per year, it should last me ages. I mean, even though it failed miserably with the first board, I still got two years use out of it before the problems started. And for these two years it was my main computer and switched on most of the time. So mathematically, if I use it one month a year with the new board it should give me another 24 years. ;-)
I'll also only use it for long periods on a cooling pad.
Even I will get bored with it by then. If I outlive it that is, because I'm not a young person.
The Mac Pro meanwhile, cost me very little as it was second-hand (£730 before I added SSD and RAM). I could get another for little money if needed. And it's in a different league. It's not faster, in fact it's a tiny bit slower when opening programs, as the SATA bus is only half as fast, but the SSD still makes a tremendous difference. But it runs much cooler than the Macbook Pro and because of that, I think it can last a very long time. And while the SATA bus is slower, I'm sure the two 2.8 GHz Xeons can outperform the single 2.2 GHz i7 quad core in the Macbook.
I'm even considering getting one of the new, dustbin model Mac Pros. But... they are seriously expensive, and the Macbook experience has made me cautious.
Ok, back to the main subject...
Apple flashed my original serial number into the new board. However, THE ETHERNET MAC ADDRESS IS DIFFERENT.
So that's a real tell-tale sign. If a logic board has been changed, the ethernet MAC address should be different.
Thanks again for the supportive replies at the time.