Hi,
I've tried to find out which Regiment William may have served with prior to the Invalid Battalion but it's like looking for a needle in a haystack
The only way I know to check for any pension or discharge papers will be via the national archive centre but without his Regiment it's going to be really difficult.
From the information I've found It seemed very common for front line soldiers to be transferred to Invalid Battalions after wars and this could fit in with William's circumstances with the Indian Mutiny having just finished in 1859.
I can give you a list of all the Regiments that served during the Indian Mutiny but William's Regiment could have been one of many (if indeed he did serve during the Mutiny).
National archive
Identifying regiments
Most records before 1873 are arranged by regiment. If you want to try to trace a soldier among the wealth of surviving records, you really do need to know which regiment he served in, especially if he was (like most soldiers) not discharged to pension.
Pension Payments
If you know the county or country in which your soldier was living between 1842 and 1862 for England and Scotland, or between 1842 and 1882 for Ireland and abroad, you may be able to find the regiment fairly easily. Between these dates there are records of payment of pensions, arranged by the district pay offices, which name the regiment served in (WO 22 , and PMG 8 for payments in Hanover). For nearly 9,000 pensioners in India, Canada and South Africa between 1772 and 1899, taken from WO 120/35 , WO 120/69 and WO 120/70.
The only way of researching would be to visit the national archive and start by checking the WO 120/35 ,WO 120/69 and WO 120/70 records which may have some information. If you visit the national archive I think you can search the records for free (but check in advance) also you may have to check lots of other records so the staff there maybe able advise you on which records to check.
My advice first is to have a really good read of the nationalarchive website which will give you information on all the records that are kept there etc. There is an ordering service but you would need to be confident of what you are ordering because they'll charge you a fee.
All the best
Adrian