TLS failure

[Colin Law]

I have a very strange problem, which may well not actually be a Ubuntu
problem, but I would be grateful for any help in diagnosing it.
When I go to https://vehicletax.service.gov.uk/ in Firefox it shows
'Performing a TLS Handshake to vehicletax.service.gov.uk' for a little
while then fails saying 'Secure Connection Failed, The connection to
the server was reset while the page was loading'.  A similar thing
happens in Google Chrome.  Also it happens on two machines, one
running Ubuntu 18.04 and the other 18.10.

However, if I connect via a VPN that I run (in the UK) then it
connects ok, which suggests it is something to do with the router or
connection to the internet.  This is supported by the fact that if I
connect my phone to the wifi it does the same thing, but if I connect
via mobile data it works ok.

I have not got any problems accessing other websites that I have
noticed.  Nothing has changed with my router (which is running LEDE)
for many months.  I do run pihole adblocker on my system but disabling
that makes no difference.

I am lost as to what to do next and would be very grateful for any suggestions.

Colin