Without seeing it for myself it's difficult to give a definitive answer. 

However, from what you've described, I would tend to agree with your operatives.  16 metres is high for a tower scaffold (if that is actually what it is), and having (from your description) one single 16 metre ladder with no landing stages is a bit "iffy" for this type of work. 

The Work at Height Regulations are very clear in stating that all work at height must be carefully planned, and this planning must include emergency arrangements.  With a single run ladder (and no landing stages) I would struggle to "get" the emergency plan from the information provided.  Who put together the RAMS and emergency plan for the work ?  Might it be worth discussing the situation with them ?

Just to clarify your other point, whilst a gin wheel might help with the lifting side of things (although a little too basic perhaps ?), if won't have a bearing on the safety (or otherwise) of your operatives from a working at height perspective, so I'd be looking at access first, and lifting/handling arrangements after the main issue is resolved if it was down to me.

Hmmm, well it's not the best scaffolding I've ever seen for a domestic property project, but it's not the worst either.  It would also have been better with mid-height staging in my opinion.  That said, it's been a while since I was involved with work in this area, and I'm not an expert in scaffolding so stand to be corrected by those more qualified than me in the construction trade.  Once upon a time, hod carriers would run up and down this sort of structure all day every day carrying hods full of bricks, but times move on don't they ?  My personal opinion is that the scaffolding should do the job, but maybe you should look at the lifting/handling element.  Is there any way they could carry lesser quantities than they currently are ? 

The link below might help you with more information

http://www.hse.gov.uk/pUbns/priced/hsg33.pdf

Edited by user 14 June 2018 08:32:21(UTC)  | Reason: Not specified

CCJ, somewhat dependent on the quantity of materials to be lifted on to the roof and also those to be removed (Not by "bombing" I hope!).

If you carry the materials up the ladder, it exacerbates the work at height risks. At the same time climbing would exacerbate the manual handling risks. Hence, for all but the smallest job some form of lifting appliance would be sensible. 

Gin wheel might be appropriate where it's just the ridge tiles to be replaced. Likely to need something more sophisticated if you wanted to replace the slope, e.g. vertical or inclined hoist. Lifting appliances introduce new issues to be taken into account in the overall balance of managing the risks.

Not clear from photo as to whether there might be gap on the inside face of the scaffold through which someone might fall. Also not clear as to how someone gets from the scaffold to the ridge. Roof ladder would be the usual solution. See HSE Guidance HSG33.

Edited by user 14 June 2018 15:04:38(UTC)  | Reason: Typo