LINEN and cotton, pure or blended, have always been favoured fabrics for spring in Hong Kong: they always look fresh and cool, and are easy to care for. These natural fibres dominate the Marks and Spencer wardrobes for men and women this spring and summer, as the style is for simple relaxed dressing. Layering is an important feature: wear short over long, or loose over fitted to create fluid, unstructured lines. Two new themes epitomise the new spring style. First is ''Sea Breeze'', which brings muted shades of navy, warm neutrals and tonal blues to make a traditionally nautical start to the season. While the lines are familiar, and classically tailored, the jackets are longer and leaner and worn with wider-leg trousers. This is the important new look in women's wear. Men's suits are more casual, with softly structured tailoring, ticking-inspired shirts and knitted silk ties. These easy, casual separates for men and women reflect a relaxed mood. Striped and checked shirts are worn over white vests. The final layer is a knitted silk or cotton waistcoat to be worn with white or blue denims. The second theme, Mystics, mixes fresh, simple pieces in natural fibres and blends. Linen is worn long, loose and layered to create a variety of looks. Long-line button-through skirts, loose, open-weave sweaters, simple unstructured painters' jackets and ankle-skimming duster coats can be worn by anyone, anywhere. Neutral shades of ecru, sand and cream match the ease of the fabric. Marks and Spencer is also making simple layers the key to men's casual wear. They should begin with a pure cotton T-shirt, add a loose-fitting chambray shirt and complete the outfit with washed twill trousers and chunky, three-eyelet nubuck boots. For lightweight casual summer dressing, wear chinos trousers. Match these with this season's soft-cotton checked shirt and tailored jacket. The ''granddad'' shirt has become the new season's ''classic'', and can be worn with everything from single-breasted soft suits to easy drawstring-waist trousers. Waistcoats are still important and add a special touch to formal wear. Tapered-leg trousers, button-down collar shirts and knitted patterned or printed textural ties update classically tailored suits with a softer, slimming silhouette. The mood it creates is relaxed and pared down for simplicity. For contrast, there are loose and fitted silhouettes. Take the layering of a rib T-shirt with a tunic top and narrow-or wide-leg trousers. Crossover garments, such as shirt jackets, jeans jerseys and knitted tailored jackets, are new elements in the casual layered style. Yellow shades extend into warm colours of spicy orange-rust, orange-reds, yellow-greens and neutrals in these garments. In summer's heat, fabric is all important and with the layering required for the season's wardrobe, Marks and Spencer has turned to natural linens in trousers and shirt-jackets with textured T-shirts and waistcoats for the layered look. These fashions can be found at the largest Marks and Spencer in town, the Ocean Centre store. which has been extended to 20,000 square feet over three floors. As well, the new For Ladies store in the Landmark is dedicated to ladies' wear, lingerie, accessories, cosmetics and toiletries.