LONDON (Reuters) - A fire at an industrial estate sent clouds of smoke billowing across London on Thursday morning.  London s fire brigade said 81 firefighters and 12 fire engines were battling the blaze at an industrial estate in Ponders End, northeast London. The smoke clouds could be seen against a clear blue sky from as far away as the Canary Wharf financial district in the east of the capital. There were no immediate reports of injuries.  