Fill in the form on mais (yes, it's SAP-crap. Self-service -> Expenses -> Create Expense Claim). You'll need a cost code and approver from someone (whoever told you to spend the money should be able to provide these). Itemise each receipt (no matter how little), print and staple the form it creates to the original receipts and shove the resulting bundle under Gaynor's door. Uni regulations here (explore menu on right). Help on mais here.
Then we wait: The process involves the claims being slowly approved by various individuals. The first approver is the one you specify in the paperwork, but there are more at different levels. If someone is away (holiday/dead/maternity...) there is no elegant fallback.
To check on the status of a claim: Self-service -> Expenses -> My Expense Claims will show something like this. "Reason" is the current status. It lists the people who have already approved the claim - so there's no point contacting them.
So no clue here on who to chase. However when there's been no action on your claim for 10 days, you get an email like htis:
Following your submission 10 days ago your e-expense claim 000xx for £yy.zz has not been actioned as yet. There may be no concern here, however to expedite your claim please contact Mrs Rita Faria Miss Laura Tomlinson Mr Elliot Rothery Mr Luke White Mrs Thamar Monet
Then you should email one / all of the people mentioned to figure out what's blocking. While all of this happens, the expense is still sitting on your credit card :( There is an option to ask for ask for an advance.