It gets complicated.
Different practices receive different payments for looking after patients. The greatest differences depend on what contract the practice has with the NHS. There are three types: GMS, PMS and APMS.
GMS practices now receive payments that are dependent on the profile of the patients: the 'value' of a patient is calculated based on some of their characteristics, for instance an older person is more likely to get ill, so the reimbursement for looking after them is higher. There is a complicated formula that attempts to make payments fair, so that payments are related to workload.
So far so good, however in 2004 almost every GMS practice (98.5%) received top-ups as pay was higher under the previous contract than the formula determined. The top-up is known as MPIG and it basically perpetuates historic unfairness: Certain practices continue to get extra money because they have always received extra money...
Since 2008 there have been changes that are trying to make the formula determine the pay and it seems locally now (2016) 'only' 64% of the practices receive more than their fair share. Plans are in place to reduce this to equity by 2020. Bearing in mind that the Global Sum value of a patient is £62, a top-up of £15 or £20 represents 25-35% extra gross pay, or 50-70% extra in net pay.
However, even if GMS practices get fair pay in 2020, a special case will continue to exist for practices that have different contracts: PMS and APMS. The unfairness in pay for GMS pales into insignificance if you look at how much more these practices receive. Different contractors receive different 'extras'. For PMS they were known as 'growth monies' and for the most recent contracts given to companies they are known as 'APMS premium'. For instance the contracts awarded for the 8-8 7/7 'equitable access' practices receive about £1 Million a year top-ups, irrespective how many patients are registered (£200 extra per patient for a 5,000 patient practice).
In moving from historic to formula-based pay, fairness for GMS practices will depend on how good the formula is at sharing out the resources. The indications are that it will be better overall, but for individual practices the formula is probably inadequate. The correction for socio-economic deprivation is probably the weakest point in the formula. It is based on the average deprivation in the area, and not at the level of need for each person. NHS Digital is now publishing payments to practices (!large file from here).
However without addressing the funding differences between the different contracting modes (GMS, PMS, APMS) real fairness will never be achieved.