1. Old copper in standard valve (8)
STOPCOCK. OPC=old copper contained in STOCK=standard.
5. Rigorous limit avoid by engineers (6)
STRICT. Remove the engineers - RE - from [re]strict=limit.
9. I'd forget to recycle sack (3,3,2)
GET RID OF. Anagram (recycle) of IDFORGET.
10 Stole what diner may leave with dog? (6)
TIPPET. Unless it's service compris, that's what you give your waiter, with the family dog. This article of clothing doesn't seem to appear in the Georgette Heyer oeuvre, probably because it came in later, in the Victorian and Edwardian eras. I just about remember an elderly female relative who had an evil-looking (and smelling) sort of fox thingy with beady glass eyes that was draped about her shoulders. It may also be a part of clerical wear, though not in fur probably.
12. Will's lost unpaid work (6,2,4)
LABOUR OF LOVE Reference to Shakespeare's Love's Labour's Lost.
15. Take notice? No point (6)
ADOPT. AD=notice. O=no. PT=point.
16. One hard poet translated is Paris prizewinner (9)
APHRODITE. Anagram (translated) of I=one with HARDPOET. In ancient mythology she competed in the beauty pageant with Hera and Athena and was awarded the golden apple by Paris as the fairest of them all. Much trouble ensued and there wasn't even a swimsuit contest.
18. Unlimited confidence in primordial bird (9)
FIRECREST. [S]ECRE[t]=confidence contained in FIRST=primordial.
19. Gather urgent request needs time (5)
PLEAT. PLEA=request with T.
20. Profitable business's general condition leading to worry (5,7)
GOING CONCERN. GOING (as in tough going)=general condition with CONCERN=worry.
24. A pound in the bank; are you sure? (6)
REALLY. A L=a pound contained in RELY=bank.
25. A number of families from America turned back at the frontier (8)
SUBORDER. A taxonomy term. SU=America reversed with BORDER. An inescably topical clue, in effect flipped on its head.
26. Shelter in squalid place, from such weather? (6)
SLEETY. Perhaps continuing the motif from the previous clue. LEE=shelter in STY.
27. Woman and writer go off to marry (8)
PENELOPE. PEN=writer with ELOPE. When I was a kid I thought that was exactly how the name was pronounced.
1. Pundit's second childhood, for one (4)
SAGE. S with AGE, a nod to Will's seven ages of man.
2. Grass is needed by horse, I swear it? (4)
OATH. OAT=grass with H for horse.
3..Rise briefly over ocean, leaving river and coming to a high point (9)
CLIMACTIC. CLIM[b]=rise briefly then drop the R from the A[r]ctic ocean.
4. Brown and black farm animal, sweetly pretty (9-3)
CHOCOLATE-BOX. CHOCOLATE=brown. B=black. OX= farm animal. It seems that once upon a time the boxes were adorned with faces of conventionally cute girls. In my childhood they seemed to come with pictures of bijou thatched cottages surrounded by hollyhocks and delphiniums. Nowadays I try to walk quickly past the Godiva display with its smart gold wrappings.
6. One to be taken by the force? (5)
THIEF. THE F containing I. A nice "and lit." (see glossary).
7. Better to conceal one's made up (10)
IMPROVISED. IMPROVED=better, contianing IS=one's.
8. With drink man would take food at hotel without ever letting up (2,3,5)
TO THE DEATH. TOT=drink. HE'D=man would. H=hotel in the NATO alphabet this time.
11. Teach recruit badly, it's a fine art (12)
ARCHITECTURE. Anagram (badly) of TEACHRECRUIT.
13. Put down sums for models (3,7)
LAY FIGURES. They're jointed human figures, often wooden, used as models by artists. LAY=put down. FIGURES=sums.
14. Full of noise, harangue in support of county match (10)
COORDINATE. CO=county and ORATE=harangue containing DIN=noise.
17. Deliberately forward contract, work being held up (2,7)
ON PURPOSE. ON=forward. PURSE=contract (as in lips), containing OP=work reversed (being held up).
21. Gold edging for English jacket (5)
GILET. GILT=gold containing E. Here they're just called vests. I believe Pip has had some experience with les gilets jaunes.
22. Tropical plant not fully bedded down (4)
EDDO. Hidden in [bedd]ED DO[wn]. Root veg used in some cookery. I don't know why I knew it - probably from the NY Times puzzles.
23. Liberal, like some public houses (4)
FREE. In other words a pub not tied to a particular brewery, but of course you knew that!