Solving time 25 minutes

Quite a lively puzzle with a wide range of clue structures that include only one anagram. There's a levening of general knowledge required but no real obscurities with the possible exception of "quint" that should be familiar to card-playing anoraks. Good to see a mention for Alan Turing, to whom a lot of people owe a great deal.

Across
1	POSTMISTRESS - POST-MI-STRESS; after motorway journey=POST MI;
9	TURIN - TURIN(g); reference the immortal Alan Turing 1912-1954 code breaker extraordinaire;
10	RIDGE-POLE - RID(G)E-POLE;
11	MANDALAY - (a)MANDA-LAY; where Kipling's road goes to;
12	QUAINT - QU(A)INT - a five card sequence in piquet is a QUINT;
13	COCK-EYED - CO(C-KEY)ED; clubs=C; COED=school;
15	LARVAE - LA(R)VA-E(ndanger);
17	AMERCE - A-MERCE(r);
18	WATERLOO - W(ATE-R-L)OO;
20	TENNER - note=TENNER (ten pounds); sounds like "tenor";
21	BOOK-REST - B(O-OK)REST; over=O; right=OK; BREST is a French naval base on the coast of Brittany;
24	CONDUCIVE - plot="connive" then replace "n"=knight by DUC=French nobleman;
25	NIECE - NI-(t)E(a)C(h)E(s); NI=Northern Ireland=province;
26	TRAIN-SPOTTER - T(RAINS-P)OTTER; an anorak is somebody who has a dull and unsociable hobby; Begbie's father's question of his son;
Down
1	POTOMAC - PO-TO-(CAM reversed);
2	SPRING-CLEANING - SPRING-C(hars)-LEANING;
3	deliberately omitted - if puzzled seek help in the usual manner;
4	SERRATED - SE(R-RAT)ED;
5	RIDE - RID-(hous)E; answer used in 10A;
6	SPECULATE - ("use past clue" without "us")*;
7	NON-INVOLVEMENT - NON-(INVOLVE(d))-MEN-T(rue); women=NON MEN;
8	GENTLE - GENT-L(ikeabl)E;
14	EXCHEQUER - EX-CHEQUER(s); Chequers is the PM's official residence at the foot of the Chilterns;
16	BARONESS - BAR-ONE-SS; start to composition=BAR ONE; ship=SS;
17	ATTACK - A-T(T)ACK;
19	OUTWEAR - OUT-WEAR; not allowed=OUT; the Wear is a well known river in Durham;
22	KENDO - hidden (bro)KEN DO(or); Japanese sword fighting;
23	GIGI - GI-GI; 1958 movie starring Leslie Caron with music by Lerner and Loewe;