Do NR have a policy of rolling upgrades, maintenance and renewal? or do they do everything (as it seems to me) piecemeal? Is this dictated by politicians or is this something internal or inherited from Railtrack/BR?
It's slowly moving to a 'schedule driven maintenance regime' from the 'demand driven maintenance regime' but given BR rushed a whole load of repairs just before privatisation with little thought for future maintenance (the WCML had no TSRs on the day it was handed over to Railtrack) and then Railtrack spent their entire existence running around in a panic botching repairs and trying to figure out what BR did and why before the whole gauge corner cracking disaster at Hatfield, it's not a massive surprise that Network Rail are still trying to work out what's gone on and why.
Network Rail also have the added trouble of having identified lots of BR-era and some Railtrack-era components - from switches and track components through to level crossing warning lights which are unreliable, dangerous or degrade and fail to meet legal requirements.
The Severn Tunnel is finally moving to a scheduled maintenance regime, so it'll be blitzed once per Control Period - new track and a visit from a ballast cleaner once per 5 years, new sleepers and complete re-ballast every 20 years, which is why it's getting new ballast, sleepers and rail at the moment. All being well, it won't need any major work until 2020.