I mix using headphones, Samson SR850s, cheap but sounds good.
Now, do I ever need studio monitors if I can mix on Headphones and play on normal speakers, hand free and mobile phone for reference?
Technically, you don't need studio monitors.
If you know what a great mix sounds like on your headphones, and you understand what mixing decisions are necessary to create a similarly great mix, then do you really need anything more? This is the underlying concept behind A/B'ing and why it's so important.
Studio monitors are a fantastic tool for creating transparent mixdowns, but you don't need them for a great mix. You need great mixing skills and solid reference points.
If you want to mix, yes, you need a pair of monitors.
The main reasons are two. Monitor speakers can handle a faster audio transition, that regular speaker can't. You can damage your hi-fi audio speakers by trying mixing drums on them for example, with permanent cone distortion damage. The second one is that headphones lie to you. You have a totally wrong audio panning image, and due to close ear distance, you hear totally wrong audio freq spectrum. With a monitor pair, in a triangular shape with your head, you will have the correct pan image, and with playing audio on low levels (so your room could not color your sound much), you will have an accurate enough freq spectrum. So, if you are serious about mixing, you need a pair, and just use your headphones for reference and checking, like you already do on your normal speakers.
If you want even more accuracy in your mixes, try room audio treatment too, you haven't to pay much for it and you can also try DIY solutions. You also don't need expensive monitor pair if your room is untreated. it's not worth it at all.
Have a nice time mixing :)