Per the UK Border Agency website, a “general visitor” (with or without visa, depending on citizenship) should not “live in the UK for extended periods through frequent or successive visits”. So if you already made several long stays and try to come back for several months, there might be a (small?) risk that the border force officer checking your passport would suspect you are lying about the purpose of your stay in the UK and are actually living there illegally.
I also remember reading about a limit of not more than six months in any twelve months but I don't know if it's a hard limit or just a rule of thumb and I can't find any official source for that. If that's the case, traveling around March should be no problem, traveling in May would be more difficult (if I understand your description correctly, at that point, you will have spent 8 months of the previous year in the UK).
I am not a lawyer but I assume that formally, as in most countries, neither a proper visa (if your situation made one necessary) nor a previous visit would entitle you to enter the UK. Border officers will typically evaluate whether your fulfill the requirements for entry every time you cross the border, which means that every time you leave the country, you expose yourself to the risk of being refused entry, however small.
In any case, if you are questioned at the border, anything you might provide to establish that you are a legitimate visitor (details and documentation of where you stayed and will be staying, details of people you want to visit or things you want to do, name of places you intend to see, funds to support yourself, a return ticket to your country of residence, etc.) might be helpful.
