3. for things like oil filled radiators in an office, what is the re-test frequency?
Whatever the test/inspection schedule for that organisation/site says !
any electrical item in an office again should be tested annually (duty of care?)
No, they should be inspected according to the inspection schedule worked out, and tested to the testing schedule. There is no set period defined - the law only says that there should be a plan in place. There are guidelines (which I can't remember) which people tend to use.
For something like a large PC under a desk, it might be a visual inspection every 2 years, and a test every 4. For something like a corded kettle (so the cord gets a lot of stick) it might be inspect and test every 6 months.
In practice, there is a tendency to "get someone in" once a year and test everything - because that's a lot easier than learning what you arer actually supposed to be doing. And of course, those charging for this often aren't keen to correct the customer and do themselves out of work.
my understanding is that the manufacturer is liable for failure within first year
Not in a commercial environment. It is determined by the contract between supplier and purchaser - the manufacturer can (and I'm dealt with this) give (say) a 30 day warranty after which any repair/replacement is down to the purchaser. Usually (especially on the sort of things that are also consumer items) there' is a sensible length of manufacturer warranty.
Under EU legislation, anything sold to a consumer (ie not business) must have a 2 year basic warranty - Apple got hauled into court in Italy over this, they were still trying to charge extra for extended warranties without letting on that their 1 year standard warranty was only half the legal minimum.
but if anyone can be certain that this isn't the case then feel free to tell us your view
I felt free