I am going to Thailand in may for 5 weeks, and I have booked my flight and accommodation for 5 weeks, I know that I only get a 30 day visa which I always complete on the flight but as I am going to Singapore for 3 days in my second week does that mean I automatically get another 30 days when I return to Bangkok, I would be grateful for any constructive advice.

Assuming you're a nationality that gets a 30 day stamp, then YES, you'll get another new 30 day stamp after entering from Singapore. The other issue is that without a visa in your passport boarding airlines on your initial flight might give you flak about your return flight being over 30 days from departure. So be prepared to show your AIR TICKET out of Thailand within 30 days of arrival and that should be good enough to allow you to board the airplane to Thailand. It must be an AIR ticket.

Ok thanks for reply, yes i am from UK so i fall into the 30 day stamp, doing it this way is less hassle than going to the Thai consulate in London and filling in forms for a 60 day one.

If you have not done it yet you should get your tickets for the flight to Singapore so that you can show it to the airline as an on onward ticket within 30 days of arrival when you check in for your flight to Thailand.