As you’ve been working there for longer than one month, then you’ll need to give them one month’s notice. You **may** be able to negotiate a shorter period, but as you’ve already agreed a start date with your new employer of 1st May, the best you could probably do is hand your notice in on Monday (26th Mar), work 2-3 weeks’ notice, then take 1-2 weeks’ unpaid leave before starting your new job. Only you can decide if leaving sooner is worth the loss of earnings.
Best to be up-front with them, in my opinion. The worst that can happen is that you have to work out a month’s notice, it will soon pass. I had to work out 10 weeks’ notice at a previous employer where I **really** wanted to leave (3 months’ notice in my contract, with my first class negotiating skills I got it down to 2.5 months). It was worth it in the end.
Good luck mate!