Hi again @myf and I appreciate all the links you sent me. The most useful one for my UT71D was the last one:
https://sigrok.org/wiki/UNI-T_UT71x_series#ProtocolWhich offers some brief information on the protocol and the VC960 link at the bottom takes you through to a page which described the protocol in more detail. This page isn't completely correct as testing showed that some of the range characters were different on my UT71D.
So to review from the start here's what I did:
1) Log data internally in the 71D on the DCV range and connected a small solar panel to the meter and pointed it towards the window for a bit less than a day.
2) Transferred the data to Termite with at UT61E serial cable ($10 from Aliexpress) with 2400,7,O,1 settings. Don't forget to click the DTR led which makes it high and supplies power to the serial lead electronics.
3) Copied the serial data from Termit and pasted it into an Openoffice spreadsheet
4) Wrote a parser for the data using spreadsheet functions such As LEFT, MID and VLOOKUP.
Piccies of the sheet are below and the columns contain the following:
A - The raw data
B - The data with leading zeroes to make it fixed length
C - The raw multimeter data (no decimal point)
D - The function selected EG DCV looked up from the tables
E - The range EG 4V looked up from the tables
F - Time - F4 = F1 then F5 =F4+$H$1/24/3600 etc.....
G - Measured Value complete with decimal point with divisor looked up in the tables.
I tested pretty much all functions, but not all ranges so there may be some errors. My AC volts test is max. 10v, DC is max 60v and frequency is max 25MHz.
All frequency measurements are normalised to Hz so they can be plotted on a graph if/when the range changes.
The data format is (mostly!)
213492501
213562501
213512501
Where digits 1-5 are the measured value, digit 6 is the range code and 7 is the function code.
I've attached some piccies of the main spreadsheet and lookup table and also the spreadsheet in .xls format if it's any use to anybody.
It's been quite a bit of work but it was fun and you will see from the graph on the piccy that it does, indeed, get dark of a night time and light in the morning in the UK. I think the little square bump is our security light turning on and off followed by a couple of car headlights.