I had a standard variable rate mortgage, do I have to go through all the initial process of fixing it?

My fixed rate interest only mortgage came to an end when the Bank of England base rate went to 0.5%. I haven't fixed it since as I was paying low payments and it has been nice because my job changed and it now pays less. Question is I know interest rates are rising and I want to get my mortgage fixed to a fixed rate mortgage for security purposes. Do I have to go through what I did when I obtained the mortgage i.e salary slips, bank statements etc etc. My job has changed and I don't earn as much anymore. Is it best to speak to my mortgage lender and get them to fix it instead?

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