I can't see your installation from here

but a lot depends on how old it is, and how well done.
On a ring circuit (if done properly) all the connections are accessible, behind the sockets, so anything which has been "crossed over" should be easy to find and fix. However, if it is an old or amateurish installation, there might be damaged cables; connections hidden away under the floor or plastered over so difficult and time consuming to find, and this is a sign of a nasty installation. If the cables are very old they may have deteriorated from age (this would not apply to 1990 cables unless they were in a very hot or otherwise hostile environment).
If you are thinking of having any domestic rewiring done, ask your electricians if they are a member of a self-certification scheme, and which one (it should be on their business cards and invoices). If not, their work will have to be inspected by the local authority building control office, which is much more trouble and expense than having it done by a suitably qualified person in the first place. It is also not a good sign.
(there are differing opinions about whether being a member of a scheme makes you a good electrician; perhaps not always; but would you want to ride in a taxi with a driver who hadn't got a licence?)
p.s. - If you had an electrical installation done a few years ago, you are not obliged to have it re-done just because recommendadtions or standards have changed, provided it is safe. "Not To Current Standards" in an information point, not one requiring urgent attention.