Front door lock

I have a front door lock which opens fine from outside but not from inside without a lot of fiddling with the key. I have soaked in WD40, there is an improvement but still not as it is from outside.
The lock is about 9 years old, and is a Willenhall 5 lever by Guardian Lock Co.
Do I need to renew the lock and if so how do I measure to ensure I get the same size?
Thanks

very likely the key is worn.

You will see it has about 8 "bits" on it arranged symmetrically. Depending which side of the door you operate from it will use either 5 bits starting from the tip, or 5 bits starting from your the loop, to lift the levers. You may see one or more of the bits is worn rounded, or possibly bent. If you have a spare key that isn't often used, you may find it is unworn and still operates the lock well and you could have it copied. If you have an ill-fitting or worn key it is no good having it copied, as it will copy the bad fit

You can buy a new BS lock for £20 - £30. For a front door you MUST get a BS lock.

They are almost all the same size - 67mm or 65mm wide, but 75mm or 80mm 'ish are also made. You can take yours out and measure it (they can vary by 2mm or so). The most important dimension is the backset from the front face to the keyhole. You may have to shave out the mortice a bit for your new lock, hopefully you will not have to move the keyhole escutcheon.

Chubb 3G114E is a very good one and has a huge guarantee. They also make cheaper ranges which I do not think are so good (as well as more expensive ones which are better but bigger and will probably not fit your existing mortice).

Wickes own-brand look surprisingly good for the low price. Lift up any you are thinking of buying, a strong solid one will be heavier than a cheap one. Cheap ones are more likely to go wrong or wear out or get a broken spring.

The BS improved a few years ago and you will find new locks have a bigger bolt.

p.s. if you are taking the lock out you can, if you want, clean it and inspect inside. Do not use oil as it will attract dirt and grit which will wear the moving parts and eventually cause it to jam. If you take it apart, I recommend taking photos as you dismantle it so you put it back together correctly. Scratch each lever you take out with a sequence number

p.p.s if your back door has a similar lock, you might consider buying a pair suited to the same key, this is very convenient and reduces the need to have extra keys or carry big bunches around

a lock that is predominantly or even exclusively locked from one side only can eventually become only functional from that side.

this has happened to me twice. the locks were not top end though.