What relates to blockages, you should always call a drain engineer, whenever you notice the smallest sign of blockage. Because, as explained above, longer you leave a blockage bigger the problem becomes.
There are many different signs of blockage. The first one that you would notice might be a foul smell coming from the drains - from the sink or bath, or toilet. It can be that the water starts draining very slowly in the sink or in the toilet bowl.
Contact Everyday Drains for any blockages.