Second-placed Newport Saracens stayed in contention with a 29-17 home win over Llantrisant while Brynmawr came close to ending their wait for first win of the season but were edged out 22-14 by Rumney.
League Three East leaders Abergavenny were made to work hard at Nelson before tries from flanker James Williams and wing Andrew Delaney went over to secure a 12-3 win. At the other end of the table, Old Illtydians beat Rhymney 20-13 to leave both sides still scrapping for survival.
Treherbert now trail early League Three South East pace-setters Ynysybwl on points difference after winning their top of the table clash 25-17 that sees both teams now locked on 54 points. Llanishen clinched the bragging right over rivals Fairwater, winning the Cardiff derby 25-17 thanks to tries from Stuart Richards, Paul Broaders, Andy Howard and Grant Morris.
Newport HSOB who blew open the title race in League Four East with a surprise 21-15 victory over leaders Garndiffaith 21-15. Gareth Marshall and Steve Jones grabbed the tries to bring the Old Boys to within three points of top side. Oakdale are still 12 points adrift at the foot of the table after they were dispatched 46-14 at Cardiff HSOB.
Penallta made their game in hand over League Four South East rivals Penarth count with a 34-7 victory over Taffs Well that opened six-point gap at the top.
League Five East title favourites Machen got back to winning ways following their first defeat of the campaign with 37-20 win over visiting Gwernyfed.
Bridgend Sports, of League Five South East produced the weekend's most emphatic display with a 79-0 triumph over hapless Deri.