Complex sentence: Proposals to modernise the system had been put forward in 1856 but were shelved due to lack of funds, but after "The Great Stink" of 1858, Parliament realised the urgency of the problem and resolved to create a modern sewerage system.
Split into simpler sentences: Proposals to modernise the sewerage system had been put forward in 1856 but were shelved due to lack of funds. However, after "The Great Stink" of 1858, Parliament realised the urgency of the problem and resolved to create a modern sewerage system.