Thanks to all three respondents. It's what I'd hoped you'd say, does make
sense - and for once is the easier of the options!

Charles F

"Dave Liquorice" wrote in message
idual.net...
On Wed, 19 Aug 2020 07:51:39 +0100, Charles F wrote:

My query is - do I just continue with an occasional extra bit of F1
added, or after ten years is it a good idea to drain, wash through with
clean water, and refill with new F1?

If it ain't broke, don't fix it!

--
Cheers
Dave.