We have been caught out recently by the closing of some Post Offices. Should have checked that it was still operating before having our post sent on. Usually I like to get there to check first. Best idea is to ring the post office and ask them to accept your 'Poste Restante' which they will keep till you collect. In this case another Post Office was operating a mile away so the other one was closed. Our post should be returned to sender eventually.
It is possible to search on the internet for Post Office addresses and phone numbers but some sites are not up to date. We have compiled our own list of Post Offices that are near the canals. See our own web site for the list which has just been updated.