Hello, if it has 'just started' to leak it could indicate a blockage further on in the system. Initially, I would try a drain unblocker chemical such as "Drain Magic" that you can get from good hardware stores. It's about �4 and you have to use it really carefully as it is very acidic. If you still get a leak after that and everything appears to be draining away then the joint needs looking at - it may simply need resealing. It is difficult to say what the cost of replacing the joints would be - take a picture and measurements to your local plumbers/builders merchants (if you have one) and they should be able to tell the price(s). Or have a look at screwfix.co.uk. Hope this helps, Andy