Arrhenius equation and activation energy

I'm doing the OCR Salters Investigation on Hydrogen Peroxide decomposition and I'm using the Arrhenius equation to calculate the activation energy for the reaction as I've experimented with different temperatures. I understand the Arrhenius equation to be:
k = Ae^(-Ea/RT) which can be rewritten as
lnk = lnA – Ea/RT
lnk = (-Ea/R * 1/T) + lnA which can be written in the form
y = mx + c

So by plotting lnk against 1/T and finding the gradient of this, I will find -Ea/R.

I did this by working out the initial rate of reactions for the different temperatures that I did (ranging from 3°C to 50°C) then using the rate equation I worked out k.

That was all fine, and yes I remembered to convert my temperatures in to Kelvin, but when I got the gradient of the line and multiplied by the gas constant R I got Ea = 1.458, whereas it should be about 75...

Anyone got any ideas where I went wrong? Help would be appreciated