Marley concrete roof tiles

Not unless they are damaged. These roof tiles are generally very easy to replace. I've done so quite a few times.

Even though you can walk on it, spread the load with a pair of battens or a crawler or whatever (that is secured so it/they can't slip.) If you have a long piece of foam rubber put that under the battens. Otherwise use a duvet. If you do walk on these tiles put the flat of your foot on the joints (overlaps) between adjacent tiles, not in the middle of a tile. If the tiles are nailed cut the nail heads off with a multitool.

Bill

My experience with pantiles is that the shallower the angle and the more they have been walked on the shorter the remaining life of the tile. The ones which failed first by spalling apart under freeze thaw frost action were on the shallowest north facing part of the roof along the line that TV engineers and chimney pot fixers walked up to the chimney apex.

Just walk on the overlaps and where the trusses or rafters are if you weigh 20 stone though.

None of them should be nailed, they just hook over the battens and interlock but beware that you won't get a colour match unless you find some secondhand ones.

also, they are now made the same colour and sprayed to get the actuak colour whereas 40 years ago the colour was part of the concrete mix.

Also, the design of the nibs has changed slightly which shouldn't be a problem.

if in doubt move some existing tiles from the rear to the front to match existing and put the new ones round the back where you won't see them.

The ridge tiles also have a different profile.