The headlap looks ok in the section of verge in your picture but the verge doesn't look properly finished. There should be another layer of slate - the 'undercloak' which is attached to the barge boards and bonded to the top layer with mortar. The detail can be seen on page 18/19 of the Welsh Slate publication at the second link.
palaceray makes a good point about the short verge slates. If they're less than 150mm the sidelap is too small, the ones in your picture look ok assuming these are 300mm wide slates.
The nails look fine, aluminium is about 20% cheaper than copper but either will do the job. Your roofer should use what he quoted for but it's a fine distinction.
Getting an expert opinion can be difficult, a chartered surveyor is the best option but they aren't all experts in roofing. The RICS website has a search facility.. http://www.ricsfirms.com/
Surveyors don't come cheap although you may get an informal opinion to help you decide what to do next. If it comes to full reports then your house insurance can help if it contains legal cover.
The CORC won't be much help, they'll tell you not to let anyone touch the roof until you've had a survey done. I'd stay away from building control, they don't offer opinions on work quality and where 25% or more of the roof has been replaced they'll point out that the whole job should be subject to their approval and certification from the outset.
For now I'd just have a conversation with the roofer along the lines that you aren't happy with the work, the Velux definitely isn't flashed correctly, the slates don't lie flat and the verges are unfinished. In fairness you should give him a chance to put any issues right. The difficulty is any solution will require a lot of work and he may be reluctant, in which case you start to think of other remedies. Of course keep a record of all conversations in case you need to refer to them later.
None of the issues should have taken the roofer by surprise, you can see right away if the rafters are uniform enough for slates and it takes only a straight edge or a piece of string to check. You see immediately if a slate will lie flat when it's placed and before nailing it down. Spanish slate has had a bad name in the past but a lot of it is very serviceable these days, there's no reason why the roofer wouldn't be able to do a reasonable job if he sourced some decent Spanish slate and sorted it properly.
palaceray makes a good point about the short verge slates. If they're less than 150mm the sidelap is too small, the ones in your picture look ok assuming these are 300mm wide slates.
The nails look fine, aluminium is about 20% cheaper than copper but either will do the job. Your roofer should use what he quoted for but it's a fine distinction.
Getting an expert opinion can be difficult, a chartered surveyor is the best option but they aren't all experts in roofing. The RICS website has a search facility.. http://www.ricsfirms.com/
Surveyors don't come cheap although you may get an informal opinion to help you decide what to do next. If it comes to full reports then your house insurance can help if it contains legal cover.
The CORC won't be much help, they'll tell you not to let anyone touch the roof until you've had a survey done. I'd stay away from building control, they don't offer opinions on work quality and where 25% or more of the roof has been replaced they'll point out that the whole job should be subject to their approval and certification from the outset.
For now I'd just have a conversation with the roofer along the lines that you aren't happy with the work, the Velux definitely isn't flashed correctly, the slates don't lie flat and the verges are unfinished. In fairness you should give him a chance to put any issues right. The difficulty is any solution will require a lot of work and he may be reluctant, in which case you start to think of other remedies. Of course keep a record of all conversations in case you need to refer to them later.
None of the issues should have taken the roofer by surprise, you can see right away if the rafters are uniform enough for slates and it takes only a straight edge or a piece of string to check. You see immediately if a slate will lie flat when it's placed and before nailing it down. Spanish slate has had a bad name in the past but a lot of it is very serviceable these days, there's no reason why the roofer wouldn't be able to do a reasonable job if he sourced some decent Spanish slate and sorted it properly.
