It's possible to be the radiator cap mate. I've had this exact problem recently mate. For me, added pressure was causing water to expand into the water bottle overflow. I tried a few things, and was head gasket. But it's a really easy job to do on the drive with minimum of tools. It takes only a few hours, including the removal of intercooler pipes and what not. But if the rad cap is broken, it won't allow water to go into the overflow. I bought a dog of a starlet and was abused. Simarly, I was finding little, or no, maintenance for it in the past 3 years. Since it's been up to me to sort out. If it's not the radiator cap, then it'll be the head gasket. Don't despair though. Easy job. Just wans some £££ and time. Can do some serious maintenance on it like, valve lapping, valve stem seals, Water pump, cam belt, tensioners, poly bush engine mounts and nice blue ASH silicon hoses on the rebuild. Or just do the basics. SKim and a head gasket. I'd say, if the heads coming off, do as much as you can. Lapping valves gives you more bhp. Valve stem seals might as well get done. It's easy enough for you to do yourself. Or cheap enough to just let a garage do. Either way, lots of information on here to help you. Hopefully it's just the rad cap. But as said, I bought a neglected starlet and that's my experience. Hopefully yours will be different.