on the first month you only have £300 in earning 5% / year, on the last month you have £3600 in earning 5%/ year , so roughly only 6 months with the full amount £3600 in @ 5% = (£3600 x 5% / year ) / 2 = £90 + a bit of compound interest - ish

or on average £1800 in for the full year - £1800 x 5% per year = £90 + a bit of compound interest on the monthly payments would get to the £97