It seems that my workhorse distro prefers chronyd to ntpd. I was wondering why my ntpd was exiting after a day or two of operation. It turns out that this was due to a clash with chronyd. After reading an article on the Internet I have accepted my distro's judgement that chronyd is an improvement on ntpd, so I have configured it to use my GNSS server in the same way, by adding a server line. Here's the output of chronyc sources -v. You can see there is a wealth of detail.

# chronyc sources -v
210 Number of sources = 17

  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current synced, '+' = combined , '-' = not combined,
| /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||      Reachability register (octal) -.           |  xxxx = adjusted offset,
||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
||                                \     |          |  zzzz = estimated error.
||                                 |    |           \
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^- ntp1.ds.network               4   6   377    32  -3854us[-3670us] +/-   37ms
^- any.time.nl                   2   6   377    36  -2552us[-2369us] +/-   56ms
^- ec2-13-55-50-68.ap-south>     3   6   377    30  +3660us[+3660us] +/-  109ms
^- ntp3.ds.network               4   6   377    35  -3247us[-3064us] +/-   36ms
^- dns01.ntl02.privatecloud>     2   6   377    33   -799us[ -615us] +/-   47ms
^- cosima.470n.act.tsgnl.co      2   6   377    34  -1030us[ -846us] +/-   61ms
^+ time.cloudflare.com           3   6   377    33  -1061us[ -877us] +/-   15ms
^- 220-158-215-20.broadband>     2   6   377    32  +9851us[  +10ms] +/-   38ms
^- ntp2.ds.network               4   6   377    32  -1467us[-1283us] +/-   37ms
^- dns01.syd01.privatecloud>     2   6   377    32  -3567us[-3382us] +/-   42ms
^- mika.470n.act.tsgnl.co        2   6   377    31  -1762us[-1762us] +/-   51ms
^- pauseq4vntp1.datamossa.io     2   6   377    30  -1399us[-1399us] +/-   42ms
^+ time.cloudflare.com           3   6   377    35  -1016us[ -833us] +/-   15ms
^+ bitburger.simonrumble.com     1   6   377    30   +676us[ +676us] +/-   13ms
^- 123.50.146.78                 2   6   337    34   -899us[ -716us] +/-  172ms
^- pauseq4vntp2.datamossa.io     2   6   377    35  -1745us[-1562us] +/-   38ms
^* rpi1.local                    1   6   377    31   +354us[ +538us] +/- 6065us

 I might be able to reduce that 6 ms estimated error by tuning.