Help me unblock my drain please

Hi .Got a blockage in a drain in my garden.The sink and washing machine both drain into this.It is in a sunken paved area and is blocked and very slow draining.The sunken area keeps getting flooded.
Is there anything i should try before calling out a plumber?
Are the chemical unblockers any good and which one should I buy?

buy yourself a set of drain rods £17.00 from here must be cheaper

You can try filling a very large bucket with water (I have one 2 foot high) and pouring it from chest height (if you can lift it) into the drain.
The height of water will create extra pressure and may dislodge the blockage. If not see if you have a local Drains R Us in the directory as they are very reasonable (around Wolverhampton they charge £25 plus vat)
When you do get it unblocked make sure you do as above every month or so and you won't get a re-occurence