Selling a doer upper: do you agree with "either renovate it ALL or do NONE of it and just sell as is?"

I'm going to be selling a house (in England) I bought 8 years ago as a "doer upper" that I have not managed to do up! While getting it valued, I asked an estate agent whether it was worth renovating any parts of it to maximise sale value. He replied, "I always tell people either do it all of it or do none of it." Which makes sense - why put money into a property you're about to sell if it won't raise the sale price by more than you put in? Especially given all the extra time / effort?

But my question is: do people here agree with that "do it all or do nothing" statement? For example, my place has a degraded flat roof that's no longer water tight, that will cost something like £3-5K to repair (plus necessary drainage repairs at the same time). It looks awful. Is there any argument for getting that done before the sale? Or given the place is still a doer-upper and most likely to be bought by a builder/renovator, would fixing the flat roof be a waste of time/money? Is that true of the rest of any property too - if selling a place in need of renovation, just sell, don't mess about!

Update: just to say, I'm not looking to fully renovate in an attempt to increase value. I've already decided I want to sell and get another place that doesn't need any / minimal work (a full renovation won't be worth my time/money). The question here is whether it's worth making relatively minor improvements to e.g. that flat roof, or not bother and just flip it as is?