Almost every elementary scientific calculator (such as needed for GCSE's!) has an hours, minutes, seconds function, which appears as a button marked with an apostrophe and a quotation mark. Costs about £7 from (for example) WH Smiths.
Not sure I'd be willing to spend that much though, especially when the sums shouldn't take more than 3 or 4 minutes anyway, using DaveyBoy's tried and tested method!