Creaking floorboards and underlay stuck?

Hi all,

Bought this flat recently (built in 1860s, by the seaside) - only downside is the floorboards creak in places and its uneven
Thought I'd have a look under the carpet since I will be replacing it anyway

im not experienced but looks like wood is pretty new??

what looks like crap underlay to me is sticking badly to the boards (you can see the marking from the roller used for the glue I guess) -- is this normal? I now need to remove the crap

Creaking - Any advice on this apart from Talc and getting screwy?

Thts hardboard I think or could be chipboard. How thick is the top layer ? 4-5mm or 18mm ?

there's a bit by the kitchen I can 'bend' up, must be closer to 5mm

Hardboard then. So you need to get that all up to check the floorboards.

Looks like they put the 4mm board all over the original/replaced boards







that lighter one must be chipboard


not sure if what im doing is all worth it, but I'm learning

Update:
taken 50% of the original floorboards up and found the water/gas/electric job done by the previous renovation guys

The source of the terrible creaking I eventually found to be two sources:
1 - old rusted nails (more like 'spikes' really) between the joists and the boards
2 - way way worse sound was coming from random 45 degree nails put in between the joists and another set of joists below that - no idea why but some of them are put in random directions and don't seem to add anything, but you can put your fingers on the nail in-between the joists and feel it creak when you put weight over the top joist.

Some nails between the joists and sub-joists are impossible to get to with a hammer or wreaking bar, so considering a multi tool or maybe just a big set of pliers if I can get one of suitable size, to cut the nail and remove a section, but would leave the rest of the nail in place

The sound is better but some way to go yet....