Solution to 13A has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
8 It goes into more than one rule (6) POLITY {POL{IT}Y}
10 Returns two-fold praise (4) LAUD<=
11 Unmanageable sappers' works, involving resistance (10) REFRAFCTORY {RE}{F{R}ACTORY}
12 Election Victory! Back to people finally! (4) VOTE {V}{TO<=}{p...lE}
13 How to get a message carried by the waves? (10) D?M?D?L?T? (Addendum - DEMODULATE [CD] - See comments)
17 Part of mega lavish celebration (4) GALA [T]
18 Medico retained by business for data repository (2-3) CD-ROM {C{DR}OM}
19 Support black sensitivity (4) BEAR {B}{EAR}
21 Travelling faster than word of mouth? (10) SUPERSONIC [CD]
23 Chance of noble backing king (4) RISK {SIR<=}{K}
24 Can a small magazine art possibly be a symbol of constitutional democracy (5,5) MAGNA CARTA {CAN+A+MAG+ART}*
28 Large, adult, male black sheep (4) LAMB &lit
29 Left ousts judge! Removal leads to poll! (8) ELECTION E(-j+l)LECTION
30 Slight wound (6) INSULT [DD]
DOWN
1 Beverage Company from California, County Los Angeles (4,4) COCA COLA {CO}{CA}{CO}{LA}
2 Focus on awfully mean, defining quality of a person (6,4) MIDDLE NAME {MIDDLE}{NAME*}
3 Reportedly procure for channel - small derivatives (2-8) BY-PRODUCTS {(~buy)BY}{PRO}{DUCT}{S}
14 Run into idle idiot (5) MORON {MO{R}ON}
15 Popular and kind of mostly modern timeless tactic (10) DEMOCRATIC {MODERn+tACTIC}*
16 Independent bills are questionable source of moral belief in individual and political freedom (10) LIBERALISM {I+BILLS+ARE}*{Mo..l}
20 Gathering up litter inside well (8) ASSEMBLY {A{MESS<=}BLY}
22 A new blue rum lacks potency (6) UNABLE {A+N+BLUE}*
25 Part of body of knowledge about Catholic revolutionary (4) NECK {K{C}EN}<=
26 Each item has a receipt (4) CHIT [T]
27 Managed King's file (4) RANK {RAN}{K}
Reference List
Sappers = RE(Royal Engineers), Resistance = r, Victory = V, Business = COM, Black = B, King = K, Left = L, Judge = J, Company = CO, County = CO, Small = S, Right = R, Acres = A, Run = R, Idle = MOON, Independent = I, New = N, Catholic = C