Hello -Newbie & Question

The alternative to paying through the nose for expensive diesel is to use a quality additive like Millers diesel treatment or Archoil. 

Before E5 came out and drove is to but E10 the difference between best fuel and crap was 6 to 7P per litre. Now the difference is 18 to 20P. The very same analogy applies to diesel. Good stuff is now ridiculously expensive. Using an additive costs a fraction. 👍👍 

I thought map updates were no longer offered by Toyota ? 

As you are probably aware, that unit has the maps on an internal hard drive, unlike earlier units that had maps on CD or the later units (touch gen 1 onwards) where it's a download onto internal flash memory. Unfortunately it's probably the most difficult unit of the lot for a DIY update.

If I remember correctly, the "official" route is via a transfer from an "upgrade box"(which plugs into the socket behind the unit's screen) at a dealer.

Updated map support dropped for these and the last maps are about 2019 vintage.

It would appear there are other ways than a dealer to upgrade, but I expect that involves opening up the unit and replacing the hard disk. Possibly only tackle if confident, good at taking things apart and putting them back together and possibly a little electronics knowledge. See:

toyotamaps.online/equipment/toyota-lexus-hdd-gen6-eu-maps

www.satnavishop.co.uk/toyota-gen6-x2808hddx29-sat-nav-map-update-europe-2019-2020-1086-p.asp

It also depends what you actually mean by "vagaries"?

Do you mean the routes you travel are not on the map (i.e. relatively new roads?)?

or

The thing generally has difficulty finding its position?

If it's the latter, a new map is unlikely to fix it. It's more likely a problem with the GPS antenna (located on a metal plate under the centre of the dash, if I remember correctly) and/or the vehicle's speed input signal into the unit.