Thanks, all grist to the mill.
I'm using the Trip Advisor forum as well as this place seems a tad quiet.

Someone in there has suggested this...

1 BHX SFO
2 San Francisco San Francisco Alcatraz
3 San Francisco Monterey (121) PCH
4 Monterey Monterey Whale cruise
5 Monterey Cambria (103) PCH
6 Cambria Furnace Creek (382) Travel
7 Furnace Creek Lone Pine (106) Death Valley
8 Lone Pine Lee Vining (122) Mono Lake & Bodie State
Park
9 Lee Vining El Portal (76) Yosemite
10 El Portal El Portal Yosemite
11 El Portal Lake Tahoe (206) Travel via Hwy 49 & 88
12 Lake Tahoe Lassen (150) Travel
13 Lassen Crescent City (330) Travel
14 Crescent City Fort Bragg (216) PCH via Hwy 101
to Hwy 1
15 Fort Bragg San Francisco (170) PCH via Hwy 101
to Hwy 1
16 San Francisco San Francisco Shopping
17 SFO BHX

Not happy about the two long travelling days but I reckon to have a day
in hand. Is there a worthwhile 2-day journey from Cambria to Furnace
Creek?

Cheers,

In the end the decision has to be yours. I reckon Cambria to Furnace Creek
is going to be quite a day. Why not see if there's somewhere (Lone Pine,
spend 2 nights there) before you get to Furnace Creek and then make a
slightly longer day in Death Valley? As for Lassen to Crescent City you
can save a lot of driving by simply going straight for the coast, crossing
I5 at Redding and then driving up the coast. Yes, you have to come back
the same way next day but I'm not sure that's a bad thing. Firstly you get
different views under different lighting conditions and, if you spot
somewhere on the way up, you don't HAVE to stop then, you can mark it for
the next day.

There's not much accommodation in Lassen itself from memory. I seem to
remember (we were coming from the north) stopping in Redding and then doing
Lassen as a day trip and spending the next night in Chico (but Chico was
dictated partly because of the university bookshop there).