Vimal wrote:

Should a chain be replaced when front chain rings are replaced?

If the chain is quite worn. Otherwise no need if the chain is still fresh
or hasn't elongated by more than 1/16" per 12 inches. Maybe 1/32" inch if
you're more fussy.

The idea that chain and sprocket wear has to be perfectly matched doesn't
make any sense when you think that with derailleur systems, some gears are
used more than others. Say you have a triple chainset and only use the
inner ring very occasionally, you wouldn't think after a while: "I better
change this ring for a more worn one so it matches the chain".

I ask
because I've replaced the chain rings and found that there is what
seems to be a regular "lag" when pedalling ... the chain seems to
take a half a turn before it takes effect.

Half a turn of what? Is there a great crunch noise and a violent skip?
If not, I suspect the chain is working well enough with the ring.

It is normal for there to be a slight lag somtimes due to the way the
(usual type of) freewheel or freehub on your rear wheel works. The pawls
have to move a certain amount before they engage with the ratchet.

There are no broken teeth
in the rear casette, which along with the chain were replaced about
100 miles ago. Is there a solution.

Need to know what the problem is first, or if there is one at all.

~PB