"Dropped yourself in it " is the wrong phrase, however the next conversation is "will you get paid to find the leak, and if not are you liable for all investigation work" I would assume not on most contacts however some HA contacts are what are commonly referred to as 3* meaning you get paid a monthly sum to look after everything so repairs and parts simply come out the grand total,
Back to basics a bit and this is where we will now argue for days on what is right and wrong,
Let's stick to the rules first then we will get onto our opinion/preferences next
When doing a TT on existing with existing appliances you do not need to isolate the appliances as it's very clearly documented in a table what you are allowed to leave depending on pipework and or meter that is fitted, assuming there's is no smell or report of smell of gas etc etc
However ( as you have done) if you choose to isolate the appliances you cannot have a leak on the pipework,
Your "mistake" for want of a better phrase at the time of finding the drop was that before you did anything else you should have discussed the options with the responsible person for the property, and this is where I KNOW the regs inside out and I'm 100% sure I will not get into any trouble by complying with "their" regs, ( a bit arrogant I know, but it's true haha)
Once you discovered the drop what I would suggest is that you discuss the options, those being
1) leave it as it's permissable ( DOCUMENT THE DROP ON YOUR PAPERWORK ) some paperwork asks for pressure at start and finish of TT and whether it's a pass or fail, some just ask for pass/fail, depending what you use write start 20mb, finish 18mb, PASS, or in the one with just pass/fail write PASS but int he notes write "2mb drop found" this is the bit that protects you, landlords won't want the leak fixed, but if it gets worse and blows up and you haven't told them about it they will greet it isn't their fault because if you told them they would have happily paid £1000's for the repair ( yes we know it's a lie, but you have let them off the hook)
The reason I document the leak in the notes is that in year one it's 2mb in year two it's 3mb if the landlord doesn't read the paperwork and notice it's getting worse that's his prob, again back to you advising him of the drop
2) price up the options for repairing the leak, which will vary depending on what is found and how long it takes to find, eg you agree to confirm the carcass, you isolate everything and find the carcass is ok, now you eliminate the appliances and find it's the tenants own cooker, they greet they can't afford a new one, or it's on the boiler and it needs to be renewed so the landlord if peeved
3) the leak is on the carcass and requires flooring,
tiles carpets Etc need to be ripped up make sure you have no liability for anything
Where I have an issue with people saying they isolate the appliances and find the leak on the carcass as "good practice" is that the tenant or landlord or both could take the engineer to task over what they have done as there is simply nothing in the regs giving authority to isolate then cap the gas for a 2mb drop on existing with existing appliances, yes I know it's done for the best etc but I'm sure some smart bum lawyer will have a go at suing a reputable, respectable gas bod for trying to protect someone
As an assessor I have stated many times I have no problem with leaving a leak if it's permissable, documented as mentioned earlier and I move on, some say they would never leave a leak and ISO every time and that's fine if you discuss the ramifications with the person paying the bill. Jeezo my fingers are sore typing this reply
In this case what happens next depends on the contract you have with the responsible person, and also the type of landlord they are, they might try to get you to fix it for nothing as you have overstepped the mark by isolating before me getting permission, the scenario I use for this to guys is
You put you van in for an MOT it passes because they have changed the brake pads as they looked worn, would you pay for the pads or refuse as they didn't discuss it or get permission to do it first, not quite the same but nearly