Let's jst get this out the way right up front. Any gas installation that's permanent in nature's gotta have a CoC, and the CoC's gotta be issued by a registered installer. Thems the law.
It's a piece of paper signed by a registered installer certifying that he/she checked your gas installation and found it to be compliant with the regulations as they stand at the moment. Currently, a CoC has to be issued from a carbon-copy CoC book issued by the gas safety association. No electronic CoC's exist. If you installer issued you one, you was robbed.
You need a CoC if you have a gas installation, it protects you from your idiot neighbors when they decide to do some dodgy home spun spoeg-n-plak gas installation on a shoe-string budget. It also breaks your insurance brokers heart when he can't refuse a claim because you're paperwork is up to date (this alone is reason to frame the thing). Also conveyancers charge per line item, so if they can add an extra line item they do, they don't really care what the paperwork is they're shifting, they just gotta shift something because line items.
You need two things.
We've got a whole lot of point #2s, but #1 is the deal breaker. The regulations are reviewed every 3 years, so even if your installation was certified and you had a compliance certificate (which you decided to feed the dog his breakfast on), there is no guarantee that the installation can be certified now. Also, bear in mind that when an installer signs off a fellow installers' installation, he/she is taking complete ownership and absolving said fellow of any responsibility. That right there is not what we call incentive. So pucker up, you shouldn't have fed the dog the old one.
Realistically, it's a crapshoot, sometimes we see 5 year old installations that are 100% certifiable (like our last president) and sometimes we see much newer installation where the client was hustled. Your mileage will vary.
If the installation is compliant, minutes. If not then it depends on what is wrong with it, but most of the time, it's a single visit to sort out the offending bits and then we can sign. So anything from hours to days. Usually less than 2 days.
There is no such thing, our industry is just not that sophisticated. But we can issue a certificate on an installation that is still under maintenance but that we know we will make compliant. We condemn the installation in the meantime and remove any way for you to connect the line up to a gas feed. This buys us time to complete while you get the certificate to feed your dog off again.
Ironically so is the thing you are paying with, you tight schmuck. Anyway, I already told you, the new installer signing off takes complete responsibility for the whole installation. Nobody's gonna sign up for that without a carrot dangling somewhere.
You can send in photos and we can usually tell with a fairly high confidence level if your installation is certifiable, but the only way to know for sure is to be there and inspect. The physical inspection has to take place, there's no getting around that.