Hi Viv,
This is good

, here is the reference of where you will probably find his service discharge document in the Kew archives. 1855 to 1872, WO97/1494, (Irw to You). They are organized first by regiment then by surname.
These are four page documents compiled by a board of officers just before a soldier is due to be discharged to ensure that all his service details are correct. In it you find details of his enlistment, where when etc. personal details ages, hair, eye and complexion colours. A detailed breakdown of his service, periods in ranks, which regiments served with. It also details service overseas, which countries and how long in these places, as he served in the Crimean war it will detail his medal entitlements for this and any other ones he might have picked up along the way.
The main thing about this document is that it opens up a host of other areas to find information, from this you will know the exact date of his discharge, so you can than look in the Pension books WO117 and see his details there, to be honest this information is taken from the discharge document though, it is still nice to have it. Benjamin will be in either WO117/6 or WO117/7 depending on what year he left.
All you need is to get to Kew or have someone look for you when they are there.
Another record is the Discription book, when a man joined an entry for him was made in this book, it gives all the details of his enlistment, where, when, by whom, occupation, but most importantly a breif description of the individual. These records would be used to aprehend him should he do a runner. From his discharge document you will know when he joined and can use this to find the correct description book.
A much bigger job for you, but something that will give you a month by month check on where he was throughout his whole army service are the Muster rolls, these are records for pay purposes, a list of everyone who was present in a given month, what extra allowances they received or deductions made. To cover his whole career though you will go through dozens of books, most will be boring lists of names, but occasionally there are some real nuggets. I have details of my GGGgD buying shirts, trousers or handkerchiefs from a dead colleagues estate.
Finally for now, from these documents you will know exactly where he was in 1851 although from the list provided by Scrimnet he should be in Ireland, the must records will tell you which place.
I will again send a personal message.
Gunner771.