That wasn't a question either...
Better to make additional contributions or not?
Yes, pay more if you can. When I left uni in 2000, I had a little over £8,000 to pay back.
I started paying them back in 2003, and continued to pay back the minimum only, which varied greatly depending on how much I got paid in any given month (for example if I had a good month with overtime they would take a shed load more).
I only finished paying them back in 2012!
You may not realise it but they have been charging you interest since the date of your first loan installment. If you have done a 3-4 year degree that interest will be piled on top, and if you have deferred, they will keep piling the interest on. Not so bad if you only owe a few grand. But if you owe £25k, you basically won't ever pay it back on minimum payments.
I would advise that you get it paid back ASAP or you will be paying through the nose in the long run. If that does not bother you, then just pay the minimum, but be prepared to have it taken out of your wages for many, many years if you owe a lot of money.
If I had my time again I would get rid of it ASAP because it took 9 years and that extra bit of disposable income was sorely needed at times.
I shudder to think the total money those with 20k, 30k, 40k, 50k etc student loans will have to pay back and how long it will take them!
For those close to paying off their loans, make sure you get the SLC to issue a stop notice to HMRC as soon as you have cleared the balance as they won't do this automatically and will keep taking your money. You do eventually get it back, but its better to avoid if you can.