I thought this was a terrific crossword, witty, entertaining and erudite. The bad news was that though I solved it I feel that sometimes I had answers without full understanding. So, more than usual, I look forward to comments and explanations from those that have them and would be good enough to post them.  
Across
1  SEANCES - amusing clue which was just the start of a really entertaining puzzle.

5  TI PST ER - which is a tier around post minus ‘o’, love. A tipster is someone who sells tips on sporting results on which one is prepared to gamble , and, therefore, loosely, a financial adviser, possibly as dodgy as a Northern Rock lending policy.

9  UNDERTONE - anagram

10 GRACE - G(ood) and race, for people.

12 ETHOLOGIST - anagram of ‘TO’ ‘LT’ ‘HIS’ and ‘EGO’

14 CYPRUS, concealed within the clue.

15 SCHE M ER - the Admiral was Scheer, and Bond

16 SICKERT - the Model referred to is the Model T, and Sicker is not so well in front. He was an EnglishImpressionist painter, and currently has an exhibition at the Courtauld, in London.

18 FULL OF THE JOYS OF SPRING20 RADIOGRAPH. The artist is RA (Royal Academy), DIOgenese was the Cynic, and RAPHael is the forepart of an artist. A radiograph is what you get when you take an X Ray.

25 I TINE RAN T - a tine is a prong of a fork, ‘ran’ is managed, and the whole infiltrating ‘IT’

26 ESS AYER  a feminine form of a profession often ends in ESS, though now Political Correctness means that actresses would rather be known as actors, goodness knows why…and an essayer is one who attempts.

27 GU TL ESS which is guess, the Americanism for ‘think’ around the letters that stand for Thallium. 

Down

1,2 SOUND AND FURY I am confused as to how all the elements of this clue come together. Maybe someone cleverer than I can explain. The long and sad tale refers to the Mouse’s tale in Alice in Wonderland, which Alice misunderstands to be a ‘tail’. The reference to “full of sound and fury (and signifying nothing)” is from Macbeth, where he describes life as  “a tale told by an idiot, full of etc.”So could someone now join those two references together?

3   This refers to a quote from W H Davies, the Tramp Poet of Newport who wrote “What is this life if, full of care, We have no time to stand and stare?”‘Many are’  corroborates with ‘c’(100) followed by ‘are’

4   SCOTTISH TERRIER

6   PI GEO NHOLE - anagram of ‘ego’ within ‘pinhole’

7   T SARIS M - Saris are the costumes and TM is Turkmenistan, which used to be part of Russia.

8   RE E NTER - renter around an ‘e’

13  GRUESOMELY - anagram of ‘me + your + legs’

16,22 “(Full of) STRANGE OATHS” is a quote from the Seven Ages of Man speech in ‘As You Like It’. ‘Oaths’ is also an anagram of Athos.

17  CAD DIES - A cad is not a gentleman, dies = succumbs, and caddies are the assistants on golf courses, who carry the player’s clubs.19  (y)OU TRAGE(dy).

23  MEET = come across, and (1 down) sounds like meat, which is a reference to Mercutio’s line ”Thy head is as fun of quarrels as an egg is full of meat”

I have a number of crossword wishes waiting to be carried out by the crossword genie and one of them is for Scorpion puzzles to appear more often. Yes, they are very tough but I find them very satisfying to solve. I’ve thoroughly enjoyed every Scorpion puzzle I’ve solved and this was no exception. Lots of great clues to savour here but, for me, 14d is simply brilliant, and brilliantly simple.