"Jiggery-Pokery" by Araucaria

Solution

The theme is BRIDGE: JIGGER is a bridge at billiards; POKER, like bridge, a card game. Variations: 1 - literal Bridges: (a) in London (b) abroad (c) translated; 2 - river with famous Bridge; 3 - charge for Bridge; 4 - Bridges "of"; 5 - Bridge = supporting structure; 6 - Bridge = card game.
Across 9 See 21 down 10 IS/ADO/RA 11 T/ALL/AHAS/SEE 12,24 L/ON/DON 13 S/ANDERS/ON 15 ASSES (pl. of AS) (Pons asinorum) 17 A/V-(s)IGN/ON(e) 18 VECCHIO (anag) 21 SIGHS (size) 23 A/L/L END/S U/P 24 See 12 26 BLACKFRIARS (flak briers) 28 EASIEST (hidden) 29 AUC/TION	Down 1 WHITE SEA 2 PRO/LONGING 3 SOYA BEAN (anag) 4 ARE/A 5 FIRST NOVEL 6 CAM/ERA 7 TO/LL 8 BA<R>NES 14 SAN/TA MAR/TA 16 S/CH<IS/MAT>IC 19 CONTRACT 20 OP/POSING 21,9 SYDNEY HARBOUR (anag) 22 SU<BL>ET 25 NOSE (Roman, high-bridged) 11 KWAI (cry/cwy)