Institution by Samuel

The theme of this puzzle is part of an institution. The wordplay in fourteen clues leads to an extra letter not to be entered into the grid; in clue order, these give a recent milestone. Four clues contain no definition; the subsidiary indication for each such clue leads to the surname of one who makes regular appearances, and the grid entry consists of their forename. The two unclued entries are to be completed with the name of the founder (two words) and a regular feature (three words). One letter is to be amended in the completed grid such that a block of cells, when taking their position in the grid into account, cryptically indicates the name of the institution; these cells are to be highlighted. Numbers in brackets refer to the lengths of grid entries.

  1    See preamble   
10    Gas European nobleman (6)
12    Honourable woman’s aware of god (5)
14    Frequently move round diminutive peaceful-ruler? (6)
15    Lavatory is close to (7)
16    Abandon weakling holding drink (3)
17    Spoil surgeon’s cat (5)
18    Immature bird’s backing good young conservative to score (6)
20    Forcibly cut short sudden emergence (7)     
23    Sailor thanks men (3)
24    African rhino’s foot was crushing elephant’s head (7)
26    Missing tresses on top, try a rug! (3)  
28    Call about ordnance, a type of explosive (7)
33    Wine topped pitch for 3D film, perhaps (6)
35    Drama’s beginning to vex aged stars (5)
36    Reggae singer has note for resistance (3)
37    Join old intelligence agency plant found underwater (7)
38    Federer’s finally gone left for the German - it sounds out (6)
40    English scrape applause (5)
41    Tree house is backwards inside (6)
42    See preamble
  1    Mugs, unsure and upset initially, could get Gaudeamus Igitur with this entreaty? (9, two words)
  2    Employment Training stales needle holders? (6)
  3    250 damage plough (3)
  4    Errant family may head off… (6)
  5    …behind old royal captured by arranger… (6)
  6    …to establish a navy (5)
  7    Indian light infantry exploit a disordered state of affairs (8)
  8    Yellow cows on unknown northern archipelago (7)
  9    Base criticise rising hairline? (4)
11    Sons each subdue ancient walrus (6)
13    Low conservative imparting motion (6)
14    Civil service uncertainty’s lifting public revenue (4)
19    Shopkeeper crazily demands topless art (9)
21    Sue tramp moving against the flow (8)
22    Colt’s worth about 250 rand (6)
25    Eagerly rally under disciple (6)
27    Universal havoc spoilt evidence (6)
29    Reserve notice to plant again (6)
30    Rector’s leaving tumbler for one who colours (4)
31    Protuberances on skull move unsteadily back to some extent (6, three words)
32    Board fortified Spanish company (6)
34    Vegetable caterpillar ate was off primarily, on the turn? (5)
36    Dutch arranged Ebony and Ivory, perhaps? (4)
39    Some analysis makes a distinctive theory (3)
To enter this competition, send your entry as an image or in list format with the highlighted entry,  to before the 8th June 2010. The first two correct entries drawn from the hat will receive a copy of  The Daily Mail Pub Quiz Book which has been donated by the publishers.