I am in my first year of phd. doing my research in multimedia analysis and fusion of video and audio. the topic is very difficult for me and I don't know where to start. I need an expert tutor who can help me to understand the algorithms and maths behind this topic. Can anyone help me to find an expert or proper tutor in London, UK?

Do any of your supervision team have expertise in this area (I hope so!) If so, they may be able to point you in the direction of people/resources which will help.

Have a look at some Masters courses/modules at your uni. Find a relevant Masters degree, contact the co-ordinator, get a list of the courses/modules that are included in the degree, ask the co-ordinator if you can sit in on the lectures for the module(s) that you need.

Each module will probably run over a couple of weeks, with lectures and practicals, so it will feel like a short-course. It should all be at the level you need too, i.e. post-graduate level.

If you're based in London, you've theoretically got access to a wide range of universities covering a whole breadth of topics and academic expertise. I'd suggest doing what Sim suggested but widening it to look at all unis in London. I know some of our PhD students had short courses and training paid for out of their allocated student budget (usually used for attending conferences, travel etc). They managed this because they had made a case to their supervisors that it was essential for their research project and academically valid.

It might also be that you need an extra academic added to your supervisory team, to supplement your current supervisors. I know of several students who needed technical expertise to do their arts-based PhD projects, so they found staff from other unis who were added to their supervisory team officially as advisors, to provide expertise that was outside what was available within our university. These advisors get paid a nominal fee for doing this and the student can draw on them for help when needed. Sometimes the requirement for an extra person becomes apparent when the project gets underway, it's not unusual at PhD level and can't always be predicted before you start.