Renew Chinese visa in China

I entered PRC with a 90 day L type entry permit (UK passport) but left for Hong Kong after 45 days. I renewed my L type in Hong Kong but they only gave me a single entry for 14 days. I am back in mainland but I need to stay 30 more days after this L type expires. Can I renew it at my local PSB? What will happen if I overstay about 25 days? I have already booked onward travel and hotels in mid March.