What should the url be and how is it related to the id?
What is needed is adding a registrantUrlBase to monuments_config.py then something like

{
    'source' : u'id',
    'dest' : u'registrant_url',
    'conv' : u'generateRegistrantUrl',
},

And if we do not have such urls (which are constructed from id's) we should close this as invalid.

Could you clarify the question please? Suggestion to close the task as invalid when the problem is obvious with bare eye appals me a little bit. In brief words the link should be rendered as it was back in 2014 or so when it still was ok. The urls parts which are not seen are oldid and useskin=monobook. The latter part is not important while the oldid (though in my opinion rcid would make more sense) is crucial. The title part should either be removed (oldid works by itself) or not percent encoded -- this should solve the problem of url taking more space than the database allows. Alternately perhaps the db could be altered to allow bigger string maximum length. The links target is a Project namespace page in ukwiki with the list in which the monument queried could be found. If you need more details -- please ask, what we want is the problem being fixed, rather than clean backlog here while the issue persists.

My mistake. I interpreted this as being about the "real source" not the source field (i.e. wiki list). Please disregard previous comment. This will likely be fixed by fixing T112460.

I'm not merging this task into that though since there might also be additional formatting issues in the html output for the api.

Ok. The links now work but the output needs to be url decoded for actual display. This is done as part of T132029