I hate to admit it but I have lived in Marple for 55 years and never swam at Marple - but would love to !
I swam at Manchester Aquatics Centre from 2001 until about 2012 and then at Romiley - I was told that Marple pool was very crowded first thing in the morning.
So 2 things.
As other people have already said 4 lanes is not enough, it should be 6 at least, but I suspect the decision has already been made.
Using Parkfield Avenue is not a good idea. Traffic of Station Road is fast and it is too close to Hollins Lane. Using the existing entrance off Stockport Road makes more sense as the traffic at the junction of Hollins Lane slow cause breaks in the traffic making it easier to get out.
Yep, it could be horrendous at times. I went regularly because it was handy for my house so I could go there at lunchtimes, but you really had to choose your time. Wednesday evenings after 8:30 would be absolutely dead, but at 7:45 you'd struggle to move, there were that many people in there. Didn't help that they usually only had one swimming lane, and the rest a free for all - swimming pools use space more efficiently when people swim in a circle.
The new pool will presumably be 25m so will be a little longer, but not much. Given capacity was an issue with the old pool, I was a little surprised to learn they were only looking at a four lane pool. Yes there will be a children's pool, but that's not much help for adult swimmers. A five lane pool (like Romiley) would only require an extra 2m in width, although how that impacts machinery and plant requirements, I don't know. It will still be the smallest pool in Stockport.
We are assuming here that by four lane they mean something a similar width to the original pool - i.e. 2m width per lane. Unless someone has decided not to use those lengths and decided four lanes means each is 2.5m wide or something!