Advice for rental increases

Tenancy Types and Management

I am in a position where I need to increase the rents for my tenant. They have not had a rental increase since they moved in in Oct 2019 which is nearly 4.5 years ago. As a result they a paying well below the market rate for the area which has gone up substantially in that time. Furthermore, I have spent a large sum of money on maintenance of this place in the last two years something around the £2.5k mark. The figure I have in mind is still roughly £160 below the market rent but would represent an increase of £120 in one go. 

I would just like some advice and guidance about how to broach such a matter. I was already planning on explaining what I have stated above, in the hope they would understand the increase but any other tips would be greatly appreciated as well as any legislative matters I would need to consider?

Many thanks.