Get your DSA form sent as soon as you can, it doesn't matter if you have started your course yet or not. It can take quite some time for thing to get into motion, so the sooner you get your DSA approaved the sooner you can get your support in place. Having said, have just found out whether not you are in receipt of DSA, you are entitled to some form of study supports, I sort of remember some people here have mentioned theya re refused help form their universities because they are waiting for their DSA to come through, well, it's apparently illegal to refuse to give study support to students... this is what I was told by the DA at my uni.
As for the needs assessment, you don't get it done at a university, you get it done at an assessment centre, or somewhere like that. Some centres are based at universities or some universities have this service, but this is not always the case. I had mine done in a very fancy co-corporate building that my assessor happens to be based at. I bet the rest of his company probably work from home. Anyway, it doesn't matter, because once your DSA is approved, you'll be sent a letter to tell you where to find your nearest assessment centre, or a centre of your choice. It's worth asking around which centre is better if you happen to be living in a big city like London or sth, because there might be many and choosing the nearest might be be the best option. I chose the nearest one and oh boy, how inexperienced that guy was? I was still emailing him a month after the assessment to have info corrected...