Hi again, and thanks to all replies so far - much appreciated and very helpful.
To answer Chris: we, too, wondered about why were were being asked to pay a deposit BEFORE the survey, but the broker - part of a large, well-established marina/workshop complex - insisted this was standard procedure, and being new to boating we queried it but felt no choice but to accept that condition. Re: the estimated cost being high we've been quoted c.£800 by an electrician regularly used by the marina who's seen what needs to be done; c. £700 labour costs (3 full days) for the windows excluding the cost of the windows (c. £850 - £1000); c: £300 to fix the plumbing and make the battery storage area safe and secure (which I could do but wouldn't be allowed to as long as the boat is on a hard standing at the marina and not in a DIY shed.)
To answer Laurie: there's no safety certificate, and no one can seem to find out when the last one was. The owner has, to all purposes, disappeared and left virtually no documentation on the history of the boat. The brokers clearly want rid of it, but they also don't want to lose too much in commission.
To answer bag'o bones: the brokers only this week sold a similar sized boat (which we saw and really didn't like in terms of layout and interior) in 'turn key' usable condition' that 'flew' through a full survey, for only c. £1500 more than our current offer - which was made subject to survey - and therefore based on what we could see (and also based on a lot of homework, seeing/comparing other boats, etc.)
To hamsterfan: I did say that the survey found that 'the hull and engine were OK, and the gas cooker worked fine!
To all...
We DO like the boat...which is why we haven't just walked away as soon as it got complicated.