19a) Divert tiny stream, not river (6) T???L?
20d) Rich? He's crazy for treasure (7) ??????H
23d) Measure litres the foolishly insert gallons (6) ?E???H
26d) Bowler exploits it, bowling in amateur session at first (4) ???S
27a) Andre's over earning money working in Fleet Street (7) E??????
Many thanks in advance for your help
19a tickle (ticker-R I think)
20d Cherish = anag. "Rich he's"
23d Length = L + anag. "Then" about g.
26d Bias = first letters of "Bowling In Amateur Session"