Do you mean a generation meter or an export meter?  There's a subtle difference.

A generation meter is between the solar inverter and the rest of the house electrics, and measures everything generated, whether it's used in the house or exported to the grid.  The generation meter belongs to whoever owns the solar panels and inverter.  It won't have an MPAN number.  It is required in order to claim FIT payments, since they are paid on generation, not export.  If the meter breaks, it's the owner's problem to get it replaced, so that they can start claiming FIT payments again.  If the system doesn't qualify for FIT payments, it's not really needed.

In most cases, the export meter is simply a register within a SMETS2 smart meter, though it was possible to get a dedicated export meter before SMETS2 meters were commonly available.  An export meter will have an MPAN.  So if you swap it, then the new meter will have to be registered against the MPAN for that supply.  The MPAN itself isn't supposed to change on replacement of a meter; the MPAN and meter serial number are held on a central database.

Agreed - just to add if it is the generation meter and it's under FIT, the customer will likely have to have their FIT certificate updated. My supplier insisted I went through that process when they noticed that the original installer had missed a digit off the meter number on the original certificate. https://mcscertified.com/consumers-communities/certificate-queries/ (there's a admin charge too).

  - Andy.

Thinking about it, if this system is 15 y.o, its almost certainly on the old FIT system so there may not be an export meter. i suppose its possible that the FIT deal may have expired, and what the lady may need is an additional export meter, which i'm guessing, they won't accept being fitted by me?

Think I'll pop in have a look, and most likely decline the work, as I'm a bit out of my depth here really. If I was 20 years younger I'd have done all the renewables courses I could, i'd have really liked to get involved, but like i say: retired and just helping out a couple of days a week here and there. 

i suppose its possible that the FIT deal may have expired

Unlikely - it normally lasts 25 years for PV (and is much better than anything on offer today, so well worth keeping).

the lady may need is an additional export meter, which i'm guessing, they won't accept being fitted by me?

Export meters are like import ones - owned by the supplier (usually implemented as part of an ordinary smart meter these days). There is an option to be paid for actual export - while remaining on the original FIT tariff for generation - and that can pay better than than the export part of the FIT scheme. Have a look for SEG - Smart Export Guarantee. But that's all handled by the customer talking to their supplier.

   - Andy.

i suppose its possible that the FIT deal may have expired

Unlikely - it normally lasts 25 years for PV (and is much better than anything on offer today, so well worth keeping).

the lady may need is an additional export meter, which i'm guessing, they won't accept being fitted by me?

Export meters are like import ones - owned by the supplier (usually implemented as part of an ordinary smart meter these days). There is an option to be paid for actual export - while remaining on the original FIT tariff for generation - and that can pay better than than the export part of the FIT scheme. Have a look for SEG - Smart Export Guarantee. But that's all handled by the customer talking to their supplier.

   - Andy.