You say the offer was subject to a satisfactory Survey.
Here's my take on the situation, and it is just my take, not any comment on the law.
The survey doesn't seem to have flagged up any major problems with the boat, there are some minor issues that affect boat safety, and the loo, and battery charger don't work.
I'd make a revised offer in writing "as a result of remedial safety works being identified on the boat" and "important domestic systems not functioning" I'd offer to complete the sale for a reduction of £X to reflect these problems (make it reasonable), or at the agreed price with these issues fixed.
Personally I'd lean towards favoring the first option if you don't trust the broker. But there's no saying you can't have your surveyor revisit to confirm completion of agreed works. If you go down the 2nd route, list the required repairs in writing, and ask them to confirm that the repairs have been done in a similar format. I'd also maybe put them on notice that if a 3rd visit from the surveyor is required bacause they haven't done the work they said they have, then you will be seeking to recoup your costs re the extra visit from the surveyor.
Finally a remark that you may not like. if this is anything approaching a full sized narrow boat 30K suggests it's more than a few years old, you can expect there to be some issues that need your attention fairly early on. take a few moments to examine your actions, and make sure you're not being a tad too fussy.
Good luck with the purchase.
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Edited to say, I've pulled out of a sale after an unfavorable survey, and the vendor wouldn't meet me 1/2 way on some rather expensive repairs. The marina (BW Sawley) had the deposit back to me by return of post. and such a shame, as the owner eventually sold the boat for 20% of the boats original asking price under what I was offering, even after the survey. he basically lost the full price of the repairs not the 1/2 I was offering.
Handy hint no 312: I always make offers to brokers in writing after I've found out what the owner will accept. All my offers are always conditional on the following.
1. A satisfactory Survey report (I would never accept a minimum value of repairs, if my surveyor feels uneasy about the boat I will consider walking)
2. A satisfactory Water trial.
When I make an offer It's because I want to buy the boat in the condition I perceve it to be in, if I can't test the micronuke, then I assume it's not working, and offer accordingly. If i make these assumptions then I inform the broker, and state that if the untried parts can be proven to work then my offer will be revised upwards accordingly.
Handy hint no 112: try this question if you're ever buying another boat through a broker.. "has this boat been surveyed before me and rejected?"