Solved next Euler task!
It is relatively simple brutal force solution but on big numbers. It took me longer than expected... My own BigNumber class fails somehow on division operation. Found two new bugs in it! have then fixed. All unit-tests were green and are even more green than ever, but solution is incorrect.
Eventually moved to System.Numerics.BigInteger (thanks .net4). Bingo!

NB: Need to return to .Division and fix it. Can run this calculation and compare result between my BigNumber and BigInteger.

btw, I made good progress since last post. I have 43 solved at the moment and coming. Somebody plays against my clock and even I am pushing problems I stay at the same rank as she or he jumps ahead.