We were going to garage her S/C but try as we might, the garage is just too short to get in and close the door. So it'll go outside the house with a cover, where there is a tree nearby, so danger of sap.

There's loads on internet but any particular one that anyone can recommend? What should we look for, any downsides?

Is a carport possible? Experience of cars kept under covers hasn’t been great as no matter how tight you can make them the wind still gets under then that causes lots of small scratches as the fabric flaps against the paint. There are also car tent type structures available, maybe even storing under a very large portable gazebo would be preferable to a car cover.
https://www.houseoftents.co.uk/Portable-Garage/ 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW

Unfortunately car port is not an option - this is off-street parking on a London residential street. Car has had a ceramic coating at some point, so somewhat scratch resistant.

Can't you extend it, that's what I did got a brickie in he extended the walls by 18" and if it's a prefab one even easier you can buy extra panels for around £100. If it ain't broken down it soon will
Pulling my Vanmaster tin tent with my 2005 RED RRS HSE TDV6
1973 Land Rover Series 3 110 safari Gone but not forgotten
1990 Discovery 200Tdi Gone thank god
1996 Discovery 300Tdi sadly missed
2003 Discovery TD5 Gone to LR heaven
2004 Discovery TD5 Gone to Aberdeen hope it likes it there
2005 RRS HSE Rimini Red (my Disco 3 in a posh frock) sadly selling
Think I see a pattern here

Council might not be happy about that, seeing as it's theirs

Might be worth an ask tho If it ain't broken down it soon will
Pulling my Vanmaster tin tent with my 2005 RED RRS HSE TDV6
1973 Land Rover Series 3 110 safari Gone but not forgotten
1990 Discovery 200Tdi Gone thank god
1996 Discovery 300Tdi sadly missed
2003 Discovery TD5 Gone to LR heaven
2004 Discovery TD5 Gone to Aberdeen hope it likes it there
2005 RRS HSE Rimini Red (my Disco 3 in a posh frock) sadly selling
Think I see a pattern here